text: // Copyright 2006 The Closure Library Authors. All Rights Reserved.
  //
  // Licensed under the Apache License, Version 2.0 (the "License");
  // you may not use this file except in compliance with the License.
  // You may obtain a copy of the License at
  //
  //      http://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing, software
  // distributed under the License is distributed on an "AS-IS" BASIS,
  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  // See the License for the specific language governing permissions and
  // limitations under the License.
  
  /**
   * @fileoverview Utilities for manipulating the browser's Document Object Model
   * Inspiration taken *heavily* from mochikit (http://mochikit.com/).
   *
   * You can use {@link goog.dom.DomHelper} to create new dom helpers that refer
   * to a different document object.  This is useful if you are working with
   * frames or multiple windows.
   *
   * @author arv@google.com (Erik Arvidsson)
   */
  
  
  // TODO(arv): Rename/refactor getTextContent and getRawTextContent. The problem
  // is that getTextContent should mimic the DOM3 textContent. We should add a
  // getInnerText (or getText) which tries to return the visible text, innerText.
  
  
  goog.provide('goog.dom');
  goog.provide('goog.dom.Appendable');
  goog.provide('goog.dom.DomHelper');
  
  goog.require('goog.array');
  goog.require('goog.asserts');
  goog.require('goog.dom.BrowserFeature');
  goog.require('goog.dom.NodeType');
  goog.require('goog.dom.TagName');
  goog.require('goog.dom.safe');
  goog.require('goog.html.SafeHtml');
  goog.require('goog.html.uncheckedconversions');
  goog.require('goog.math.Coordinate');
  goog.require('goog.math.Size');
  goog.require('goog.object');
  goog.require('goog.string');
  goog.require('goog.string.Unicode');
  goog.require('goog.userAgent');
  
  
  /**
   * @define {boolean} Whether we know at compile time that the browser is in
   * quirks mode.
   */
  goog.define('goog.dom.ASSUME_QUIRKS_MODE', false);
  
  
  /**
   * @define {boolean} Whether we know at compile time that the browser is in
   * standards compliance mode.
   */
  goog.define('goog.dom.ASSUME_STANDARDS_MODE', false);
  
  
  /**
   * Whether we know the compatibility mode at compile time.
   * @type {boolean}
   * @private
   */
  goog.dom.COMPAT_MODE_KNOWN_ =
      goog.dom.ASSUME_QUIRKS_MODE || goog.dom.ASSUME_STANDARDS_MODE;
  
  
  /**
   * Gets the DomHelper object for the document where the element resides.
   * @param {(Node|Window)=} opt_element If present, gets the DomHelper for this
   *     element.
   * @return {!goog.dom.DomHelper} The DomHelper.
   */
  goog.dom.getDomHelper = function(opt_element) {
    return opt_element ?
        new goog.dom.DomHelper(goog.dom.getOwnerDocument(opt_element)) :
        (goog.dom.defaultDomHelper_ ||
         (goog.dom.defaultDomHelper_ = new goog.dom.DomHelper()));
  };
  
  
  /**
   * Cached default DOM helper.
   * @type {!goog.dom.DomHelper|undefined}
   * @private
   */
  goog.dom.defaultDomHelper_;
  
  
  /**
   * Gets the document object being used by the dom library.
   * @return {!Document} Document object.
   */
  goog.dom.getDocument = function() {
    return document;
  };
  
  
  /**
   * Gets an element from the current document by element id.
   *
   * If an Element is passed in, it is returned.
   *
   * @param {string|Element} element Element ID or a DOM node.
   * @return {Element} The element with the given ID, or the node passed in.
   */
  goog.dom.getElement = function(element) {
    return goog.dom.getElementHelper_(document, element);
  };
  
  
  /**
   * Gets an element by id from the given document (if present).
   * If an element is given, it is returned.
   * @param {!Document} doc
   * @param {string|Element} element Element ID or a DOM node.
   * @return {Element} The resulting element.
   * @private
   */
  goog.dom.getElementHelper_ = function(doc, element) {
    return goog.isString(element) ? doc.getElementById(element) : element;
  };
  
  
  /**
   * Gets an element by id, asserting that the element is found.
   *
   * This is used when an element is expected to exist, and should fail with
   * an assertion error if it does not (if assertions are enabled).
   *
   * @param {string} id Element ID.
   * @return {!Element} The element with the given ID, if it exists.
   */
  goog.dom.getRequiredElement = function(id) {
    return goog.dom.getRequiredElementHelper_(document, id);
  };
  
  
  /**
   * Helper function for getRequiredElementHelper functions, both static and
   * on DomHelper.  Asserts the element with the given id exists.
   * @param {!Document} doc
   * @param {string} id
   * @return {!Element} The element with the given ID, if it exists.
   * @private
   */
  goog.dom.getRequiredElementHelper_ = function(doc, id) {
    // To prevent users passing in Elements as is permitted in getElement().
    goog.asserts.assertString(id);
    var element = goog.dom.getElementHelper_(doc, id);
    element =
        goog.asserts.assertElement(element, 'No element found with id: ' + id);
    return element;
  };
  
  
  /**
   * Alias for getElement.
   * @param {string|Element} element Element ID or a DOM node.
   * @return {Element} The element with the given ID, or the node passed in.
   * @deprecated Use {@link goog.dom.getElement} instead.
   */
  goog.dom.$ = goog.dom.getElement;
  
  
  /**
   * Gets elements by tag name.
   * @param {goog.dom.TagName} tagName
   * @param {(!Document|!Element)=} opt_parent Parent element or document where to
   *     look for elements. Defaults to document.
   * @return {!NodeList<!Element>}
   */
  goog.dom.getElementsByTagName = function(tagName, opt_parent) {
    var parent = opt_parent || document;
    return parent.getElementsByTagName(tagName);
  };
  
  
  /**
   * Looks up elements by both tag and class name, using browser native functions
   * ({@code querySelectorAll}, {@code getElementsByTagName} or
   * {@code getElementsByClassName}) where possible. This function
   * is a useful, if limited, way of collecting a list of DOM elements
   * with certain characteristics.  {@code goog.dom.query} offers a
   * more powerful and general solution which allows matching on CSS3
   * selector expressions, but at increased cost in code size. If all you
   * need is particular tags belonging to a single class, this function
   * is fast and sleek.
   *
   * Note that tag names are case sensitive in the SVG namespace, and this
   * function converts opt_tag to uppercase for comparisons. For queries in the
   * SVG namespace you should use querySelector or querySelectorAll instead.
   * https://bugzilla.mozilla.org/show_bug.cgi?id=963870
   * https://bugs.webkit.org/show_bug.cgi?id=83438
   *
   * @see {goog.dom.query}
   *
   * @param {?string=} opt_tag Element tag name.
   * @param {?string=} opt_class Optional class name.
   * @param {(Document|Element)=} opt_el Optional element to look in.
   * @return {!IArrayLike<!Element>} Array-like list of elements (only a length
   *     property and numerical indices are guaranteed to exist).
   */
  goog.dom.getElementsByTagNameAndClass = function(opt_tag, opt_class, opt_el) {
    return goog.dom.getElementsByTagNameAndClass_(
        document, opt_tag, opt_class, opt_el);
  };
  
  
  /**
   * Returns a static, array-like list of the elements with the provided
   * className.
   * @see {goog.dom.query}
   * @param {string} className the name of the class to look for.
   * @param {(Document|Element)=} opt_el Optional element to look in.
   * @return {!IArrayLike<!Element>} The items found with the class name provided.
   */
  goog.dom.getElementsByClass = function(className, opt_el) {
    var parent = opt_el || document;
    if (goog.dom.canUseQuerySelector_(parent)) {
      return parent.querySelectorAll('.' + className);
    }
    return goog.dom.getElementsByTagNameAndClass_(
        document, '*', className, opt_el);
  };
  
  
  /**
   * Returns the first element with the provided className.
   * @see {goog.dom.query}
   * @param {string} className the name of the class to look for.
   * @param {Element|Document=} opt_el Optional element to look in.
   * @return {Element} The first item with the class name provided.
   */
  goog.dom.getElementByClass = function(className, opt_el) {
    var parent = opt_el || document;
    var retVal = null;
    if (parent.getElementsByClassName) {
      retVal = parent.getElementsByClassName(className)[0];
    } else if (goog.dom.canUseQuerySelector_(parent)) {
      retVal = parent.querySelector('.' + className);
    } else {
      retVal = goog.dom.getElementsByTagNameAndClass_(
          document, '*', className, opt_el)[0];
    }
    return retVal || null;
  };
  
  
  /**
   * Ensures an element with the given className exists, and then returns the
   * first element with the provided className.
   * @see {goog.dom.query}
   * @param {string} className the name of the class to look for.
   * @param {!Element|!Document=} opt_root Optional element or document to look
   *     in.
   * @return {!Element} The first item with the class name provided.
   * @throws {goog.asserts.AssertionError} Thrown if no element is found.
   */
  goog.dom.getRequiredElementByClass = function(className, opt_root) {
    var retValue = goog.dom.getElementByClass(className, opt_root);
    return goog.asserts.assert(
        retValue, 'No element found with className: ' + className);
  };
  
  
  /**
   * Prefer the standardized (http://www.w3.org/TR/selectors-api/), native and
   * fast W3C Selectors API.
   * @param {!(Element|Document)} parent The parent document object.
   * @return {boolean} whether or not we can use parent.querySelector* APIs.
   * @private
   */
  goog.dom.canUseQuerySelector_ = function(parent) {
    return !!(parent.querySelectorAll && parent.querySelector);
  };
  
  
  /**
   * Helper for {@code getElementsByTagNameAndClass}.
   * @param {!Document} doc The document to get the elements in.
   * @param {?string=} opt_tag Element tag name.
   * @param {?string=} opt_class Optional class name.
   * @param {(Document|Element)=} opt_el Optional element to look in.
   * @return {!IArrayLike<!Element>} Array-like list of elements (only a length
   *     property and numerical indices are guaranteed to exist).
   * @private
   */
  goog.dom.getElementsByTagNameAndClass_ = function(
      doc, opt_tag, opt_class, opt_el) {
    var parent = opt_el || doc;
    var tagName = (opt_tag && opt_tag != '*') ? opt_tag.toUpperCase() : '';
  
    if (goog.dom.canUseQuerySelector_(parent) && (tagName || opt_class)) {
      var query = tagName + (opt_class ? '.' + opt_class : '');
      return parent.querySelectorAll(query);
    }
  
    // Use the native getElementsByClassName if available, under the assumption
    // that even when the tag name is specified, there will be fewer elements to
    // filter through when going by class than by tag name
    if (opt_class && parent.getElementsByClassName) {
      var els = parent.getElementsByClassName(opt_class);
  
      if (tagName) {
        var arrayLike = {};
        var len = 0;
  
        // Filter for specific tags if requested.
        for (var i = 0, el; el = els[i]; i++) {
          if (tagName == el.nodeName) {
            arrayLike[len++] = el;
          }
        }
        arrayLike.length = len;
  
        return /** @type {!IArrayLike<!Element>} */ (arrayLike);
      } else {
        return els;
      }
    }
  
    var els = parent.getElementsByTagName(tagName || '*');
  
    if (opt_class) {
      var arrayLike = {};
      var len = 0;
      for (var i = 0, el; el = els[i]; i++) {
        var className = el.className;
        // Check if className has a split function since SVG className does not.
        if (typeof className.split == 'function' &&
            goog.array.contains(className.split(/\s+/), opt_class)) {
          arrayLike[len++] = el;
        }
      }
      arrayLike.length = len;
      return /** @type {!IArrayLike<!Element>} */ (arrayLike);
    } else {
      return els;
    }
  };
  
  
  /**
   * Alias for {@code getElementsByTagNameAndClass}.
   * @param {?string=} opt_tag Element tag name.
   * @param {?string=} opt_class Optional class name.
   * @param {Element=} opt_el Optional element to look in.
   * @return {!IArrayLike<!Element>} Array-like list of elements (only a length
   *     property and numerical indices are guaranteed to exist).
   * @deprecated Use {@link goog.dom.getElementsByTagNameAndClass} instead.
   */
  goog.dom.$$ = goog.dom.getElementsByTagNameAndClass;
  
  
  /**
   * Sets multiple properties, and sometimes attributes, on an element. Note that
   * properties are simply object properties on the element instance, while
   * attributes are visible in the DOM. Many properties map to attributes with the
   * same names, some with different names, and there are also unmappable cases.
   *
   * This method sets properties by default (which means that custom attributes
   * are not supported). These are the exeptions (some of which is legacy):
   * - "style": Even though this is an attribute name, it is translated to a
   *   property, "style.cssText". Note that this property sanitizes and formats
   *   its value, unlike the attribute.
   * - "class": This is an attribute name, it is translated to the "className"
   *   property.
   * - "for": This is an attribute name, it is translated to the "htmlFor"
   *   property.
   * - Entries in {@see goog.dom.DIRECT_ATTRIBUTE_MAP_} are set as attributes,
   *   this is probably due to browser quirks.
   * - "aria-*", "data-*": Always set as attributes, they have no property
   *   counterparts.
   *
   * @param {Element} element DOM node to set properties on.
   * @param {Object} properties Hash of property:value pairs.
   */
  goog.dom.setProperties = function(element, properties) {
    goog.object.forEach(properties, function(val, key) {
      if (key == 'style') {
        element.style.cssText = val;
      } else if (key == 'class') {
        element.className = val;
      } else if (key == 'for') {
        element.htmlFor = val;
      } else if (goog.dom.DIRECT_ATTRIBUTE_MAP_.hasOwnProperty(key)) {
        element.setAttribute(goog.dom.DIRECT_ATTRIBUTE_MAP_[key], val);
      } else if (
          goog.string.startsWith(key, 'aria-') ||
          goog.string.startsWith(key, 'data-')) {
        element.setAttribute(key, val);
      } else {
        element[key] = val;
      }
    });
  };
  
  
  /**
   * Map of attributes that should be set using
   * element.setAttribute(key, val) instead of element[key] = val.  Used
   * by goog.dom.setProperties.
   *
   * @private {!Object<string, string>}
   * @const
   */
  goog.dom.DIRECT_ATTRIBUTE_MAP_ = {
    'cellpadding': 'cellPadding',
    'cellspacing': 'cellSpacing',
    'colspan': 'colSpan',
    'frameborder': 'frameBorder',
    'height': 'height',
    'maxlength': 'maxLength',
    'nonce': 'nonce',
    'role': 'role',
    'rowspan': 'rowSpan',
    'type': 'type',
    'usemap': 'useMap',
    'valign': 'vAlign',
    'width': 'width'
  };
  
  
  /**
   * Gets the dimensions of the viewport.
   *
   * Gecko Standards mode:
   * docEl.clientWidth  Width of viewport excluding scrollbar.
   * win.innerWidth     Width of viewport including scrollbar.
   * body.clientWidth   Width of body element.
   *
   * docEl.clientHeight Height of viewport excluding scrollbar.
   * win.innerHeight    Height of viewport including scrollbar.
   * body.clientHeight  Height of document.
   *
   * Gecko Backwards compatible mode:
   * docEl.clientWidth  Width of viewport excluding scrollbar.
   * win.innerWidth     Width of viewport including scrollbar.
   * body.clientWidth   Width of viewport excluding scrollbar.
   *
   * docEl.clientHeight Height of document.
   * win.innerHeight    Height of viewport including scrollbar.
   * body.clientHeight  Height of viewport excluding scrollbar.
   *
   * IE6/7 Standards mode:
   * docEl.clientWidth  Width of viewport excluding scrollbar.
   * win.innerWidth     Undefined.
   * body.clientWidth   Width of body element.
   *
   * docEl.clientHeight Height of viewport excluding scrollbar.
   * win.innerHeight    Undefined.
   * body.clientHeight  Height of document element.
   *
   * IE5 + IE6/7 Backwards compatible mode:
   * docEl.clientWidth  0.
   * win.innerWidth     Undefined.
   * body.clientWidth   Width of viewport excluding scrollbar.
   *
   * docEl.clientHeight 0.
   * win.innerHeight    Undefined.
   * body.clientHeight  Height of viewport excluding scrollbar.
   *
   * Opera 9 Standards and backwards compatible mode:
   * docEl.clientWidth  Width of viewport excluding scrollbar.
   * win.innerWidth     Width of viewport including scrollbar.
   * body.clientWidth   Width of viewport excluding scrollbar.
   *
   * docEl.clientHeight Height of document.
   * win.innerHeight    Height of viewport including scrollbar.
   * body.clientHeight  Height of viewport excluding scrollbar.
   *
   * WebKit:
   * Safari 2
   * docEl.clientHeight Same as scrollHeight.
   * docEl.clientWidth  Same as innerWidth.
   * win.innerWidth     Width of viewport excluding scrollbar.
   * win.innerHeight    Height of the viewport including scrollbar.
   * frame.innerHeight  Height of the viewport exluding scrollbar.
   *
   * Safari 3 (tested in 522)
   *
   * docEl.clientWidth  Width of viewport excluding scrollbar.
   * docEl.clientHeight Height of viewport excluding scrollbar in strict mode.
   * body.clientHeight  Height of viewport excluding scrollbar in quirks mode.
   *
   * @param {Window=} opt_window Optional window element to test.
   * @return {!goog.math.Size} Object with values 'width' and 'height'.
   */
  goog.dom.getViewportSize = function(opt_window) {
    // TODO(arv): This should not take an argument
    return goog.dom.getViewportSize_(opt_window || window);
  };
  
  
  /**
   * Helper for {@code getViewportSize}.
   * @param {Window} win The window to get the view port size for.
   * @return {!goog.math.Size} Object with values 'width' and 'height'.
   * @private
   */
  goog.dom.getViewportSize_ = function(win) {
    var doc = win.document;
    var el = goog.dom.isCss1CompatMode_(doc) ? doc.documentElement : doc.body;
    return new goog.math.Size(el.clientWidth, el.clientHeight);
  };
  
  
  /**
   * Calculates the height of the document.
   *
   * @return {number} The height of the current document.
   */
  goog.dom.getDocumentHeight = function() {
    return goog.dom.getDocumentHeight_(window);
  };
  
  /**
   * Calculates the height of the document of the given window.
   *
   * @param {!Window} win The window whose document height to retrieve.
   * @return {number} The height of the document of the given window.
   */
  goog.dom.getDocumentHeightForWindow = function(win) {
    return goog.dom.getDocumentHeight_(win);
  };
  
  /**
   * Calculates the height of the document of the given window.
   *
   * Function code copied from the opensocial gadget api:
   *   gadgets.window.adjustHeight(opt_height)
   *
   * @private
   * @param {!Window} win The window whose document height to retrieve.
   * @return {number} The height of the document of the given window.
   */
  goog.dom.getDocumentHeight_ = function(win) {
    // NOTE(eae): This method will return the window size rather than the document
    // size in webkit quirks mode.
    var doc = win.document;
    var height = 0;
  
    if (doc) {
      // Calculating inner content height is hard and different between
      // browsers rendering in Strict vs. Quirks mode.  We use a combination of
      // three properties within document.body and document.documentElement:
      // - scrollHeight
      // - offsetHeight
      // - clientHeight
      // These values differ significantly between browsers and rendering modes.
      // But there are patterns.  It just takes a lot of time and persistence
      // to figure out.
  
      var body = doc.body;
      var docEl = /** @type {!HTMLElement} */ (doc.documentElement);
      if (!(docEl && body)) {
        return 0;
      }
  
      // Get the height of the viewport
      var vh = goog.dom.getViewportSize_(win).height;
      if (goog.dom.isCss1CompatMode_(doc) && docEl.scrollHeight) {
        // In Strict mode:
        // The inner content height is contained in either:
        //    document.documentElement.scrollHeight
        //    document.documentElement.offsetHeight
        // Based on studying the values output by different browsers,
        // use the value that's NOT equal to the viewport height found above.
        height =
            docEl.scrollHeight != vh ? docEl.scrollHeight : docEl.offsetHeight;
      } else {
        // In Quirks mode:
        // documentElement.clientHeight is equal to documentElement.offsetHeight
        // except in IE.  In most browsers, document.documentElement can be used
        // to calculate the inner content height.
        // However, in other browsers (e.g. IE), document.body must be used
        // instead.  How do we know which one to use?
        // If document.documentElement.clientHeight does NOT equal
        // document.documentElement.offsetHeight, then use document.body.
        var sh = docEl.scrollHeight;
        var oh = docEl.offsetHeight;
        if (docEl.clientHeight != oh) {
          sh = body.scrollHeight;
          oh = body.offsetHeight;
        }
  
        // Detect whether the inner content height is bigger or smaller
        // than the bounding box (viewport).  If bigger, take the larger
        // value.  If smaller, take the smaller value.
        if (sh > vh) {
          // Content is larger
          height = sh > oh ? sh : oh;
        } else {
          // Content is smaller
          height = sh < oh ? sh : oh;
        }
      }
    }
  
    return height;
  };
  
  
  /**
   * Gets the page scroll distance as a coordinate object.
   *
   * @param {Window=} opt_window Optional window element to test.
   * @return {!goog.math.Coordinate} Object with values 'x' and 'y'.
   * @deprecated Use {@link goog.dom.getDocumentScroll} instead.
   */
  goog.dom.getPageScroll = function(opt_window) {
    var win = opt_window || goog.global || window;
    return goog.dom.getDomHelper(win.document).getDocumentScroll();
  };
  
  
  /**
   * Gets the document scroll distance as a coordinate object.
   *
   * @return {!goog.math.Coordinate} Object with values 'x' and 'y'.
   */
  goog.dom.getDocumentScroll = function() {
    return goog.dom.getDocumentScroll_(document);
  };
  
  
  /**
   * Helper for {@code getDocumentScroll}.
   *
   * @param {!Document} doc The document to get the scroll for.
   * @return {!goog.math.Coordinate} Object with values 'x' and 'y'.
   * @private
   */
  goog.dom.getDocumentScroll_ = function(doc) {
    var el = goog.dom.getDocumentScrollElement_(doc);
    var win = goog.dom.getWindow_(doc);
    if (goog.userAgent.IE && goog.userAgent.isVersionOrHigher('10') &&
        win.pageYOffset != el.scrollTop) {
      // The keyboard on IE10 touch devices shifts the page using the pageYOffset
      // without modifying scrollTop. For this case, we want the body scroll
      // offsets.
      return new goog.math.Coordinate(el.scrollLeft, el.scrollTop);
    }
    return new goog.math.Coordinate(
        win.pageXOffset || el.scrollLeft, win.pageYOffset || el.scrollTop);
  };
  
  
  /**
   * Gets the document scroll element.
   * @return {!Element} Scrolling element.
   */
  goog.dom.getDocumentScrollElement = function() {
    return goog.dom.getDocumentScrollElement_(document);
  };
  
  
  /**
   * Helper for {@code getDocumentScrollElement}.
   * @param {!Document} doc The document to get the scroll element for.
   * @return {!Element} Scrolling element.
   * @private
   */
  goog.dom.getDocumentScrollElement_ = function(doc) {
    // Old WebKit needs body.scrollLeft in both quirks mode and strict mode. We
    // also default to the documentElement if the document does not have a body
    // (e.g. a SVG document).
    // Uses http://dev.w3.org/csswg/cssom-view/#dom-document-scrollingelement to
    // avoid trying to guess about browser behavior from the UA string.
    if (doc.scrollingElement) {
      return doc.scrollingElement;
    }
    if (!goog.userAgent.WEBKIT && goog.dom.isCss1CompatMode_(doc)) {
      return doc.documentElement;
    }
    return doc.body || doc.documentElement;
  };
  
  
  /**
   * Gets the window object associated with the given document.
   *
   * @param {Document=} opt_doc  Document object to get window for.
   * @return {!Window} The window associated with the given document.
   */
  goog.dom.getWindow = function(opt_doc) {
    // TODO(arv): This should not take an argument.
    return opt_doc ? goog.dom.getWindow_(opt_doc) : window;
  };
  
  
  /**
   * Helper for {@code getWindow}.
   *
   * @param {!Document} doc  Document object to get window for.
   * @return {!Window} The window associated with the given document.
   * @private
   */
  goog.dom.getWindow_ = function(doc) {
    return doc.parentWindow || doc.defaultView;
  };
  
  
  // TODO(jakubvrana): Allow only members of goog.dom.TagName in tagName.
  /**
   * Returns a dom node with a set of attributes.  This function accepts varargs
   * for subsequent nodes to be added.  Subsequent nodes will be added to the
   * first node as childNodes.
   *
   * So:
   * <code>createDom(goog.dom.TagName.DIV, null, createDom(goog.dom.TagName.P), createDom(goog.dom.TagName.P));</code>
   * would return a div with two child paragraphs
   *
   * For passing properties, please see {@link goog.dom.setProperties} for more
   * information.
   *
   * @param {string} tagName Tag to create.
   * @param {(Object|Array<string>|string)=} opt_properties If object, then a map
   *     of name-value pairs for properties. If a string, then this is the
   *     className of the new element. If an array, the elements will be joined
   *     together as the className of the new element.
   * @param {...(Object|string|Array|NodeList)} var_args Further DOM nodes or
   *     strings for text nodes. If one of the var_args is an array or NodeList,
   *     its elements will be added as childNodes instead.
   * @return {!Element} Reference to a DOM node.
   */
  goog.dom.createDom = function(tagName, opt_properties, var_args) {
    return goog.dom.createDom_(document, arguments);
  };
  
  
  /**
   * Helper for {@code createDom}.
   * @param {!Document} doc The document to create the DOM in.
   * @param {!Arguments} args Argument object passed from the callers. See
   *     {@code goog.dom.createDom} for details.
   * @return {!Element} Reference to a DOM node.
   * @private
   */
  goog.dom.createDom_ = function(doc, args) {
    var tagName = String(args[0]);
    var attributes = args[1];
  
    // Internet Explorer is dumb:
    // name: https://msdn.microsoft.com/en-us/library/ms534184(v=vs.85).aspx
    // type: https://msdn.microsoft.com/en-us/library/ms534700(v=vs.85).aspx
    // Also does not allow setting of 'type' attribute on 'input' or 'button'.
    if (!goog.dom.BrowserFeature.CAN_ADD_NAME_OR_TYPE_ATTRIBUTES && attributes &&
        (attributes.name || attributes.type)) {
      var tagNameArr = ['<', tagName];
      if (attributes.name) {
        tagNameArr.push(' name="', goog.string.htmlEscape(attributes.name), '"');
      }
      if (attributes.type) {
        tagNameArr.push(' type="', goog.string.htmlEscape(attributes.type), '"');
  
        // Clone attributes map to remove 'type' without mutating the input.
        var clone = {};
        goog.object.extend(clone, attributes);
  
        // JSCompiler can't see how goog.object.extend added this property,
        // because it was essentially added by reflection.
        // So it needs to be quoted.
        delete clone['type'];
  
        attributes = clone;
      }
      tagNameArr.push('>');
      tagName = tagNameArr.join('');
    }
  
    var element = doc.createElement(tagName);
  
    if (attributes) {
      if (goog.isString(attributes)) {
        element.className = attributes;
      } else if (goog.isArray(attributes)) {
        element.className = attributes.join(' ');
      } else {
        goog.dom.setProperties(element, attributes);
      }
    }
  
    if (args.length > 2) {
      goog.dom.append_(doc, element, args, 2);
    }
  
    return element;
  };
  
  
  /**
   * Appends a node with text or other nodes.
   * @param {!Document} doc The document to create new nodes in.
   * @param {!Node} parent The node to append nodes to.
   * @param {!Arguments} args The values to add. See {@code goog.dom.append}.
   * @param {number} startIndex The index of the array to start from.
   * @private
   */
  goog.dom.append_ = function(doc, parent, args, startIndex) {
    function childHandler(child) {
      // TODO(user): More coercion, ala MochiKit?
      if (child) {
        parent.appendChild(
            goog.isString(child) ? doc.createTextNode(child) : child);
      }
    }
  
    for (var i = startIndex; i < args.length; i++) {
      var arg = args[i];
      // TODO(attila): Fix isArrayLike to return false for a text node.
      if (goog.isArrayLike(arg) && !goog.dom.isNodeLike(arg)) {
        // If the argument is a node list, not a real array, use a clone,
        // because forEach can't be used to mutate a NodeList.
        goog.array.forEach(
            goog.dom.isNodeList(arg) ? goog.array.toArray(arg) : arg,
            childHandler);
      } else {
        childHandler(arg);
      }
    }
  };
  
  
  /**
   * Alias for {@code createDom}.
   * @param {string} tagName Tag to create.
   * @param {(string|Object)=} opt_properties If object, then a map of name-value
   *     pairs for properties. If a string, then this is the className of the new
   *     element.
   * @param {...(Object|string|Array|NodeList)} var_args Further DOM nodes or
   *     strings for text nodes. If one of the var_args is an array, its
   *     children will be added as childNodes instead.
   * @return {!Element} Reference to a DOM node.
   * @deprecated Use {@link goog.dom.createDom} instead.
   */
  goog.dom.$dom = goog.dom.createDom;
  
  
  /**
   * Creates a new element.
   * @param {string} name Tag name.
   * @return {!Element} The new element.
   */
  goog.dom.createElement = function(name) {
    return document.createElement(String(name));
  };
  
  
  /**
   * Creates a new text node.
   * @param {number|string} content Content.
   * @return {!Text} The new text node.
   */
  goog.dom.createTextNode = function(content) {
    return document.createTextNode(String(content));
  };
  
  
  /**
   * Create a table.
   * @param {number} rows The number of rows in the table.  Must be >= 1.
   * @param {number} columns The number of columns in the table.  Must be >= 1.
   * @param {boolean=} opt_fillWithNbsp If true, fills table entries with
   *     {@code goog.string.Unicode.NBSP} characters.
   * @return {!Element} The created table.
   */
  goog.dom.createTable = function(rows, columns, opt_fillWithNbsp) {
    // TODO(mlourenco): Return HTMLTableElement, also in prototype function.
    // Callers need to be updated to e.g. not assign numbers to table.cellSpacing.
    return goog.dom.createTable_(document, rows, columns, !!opt_fillWithNbsp);
  };
  
  
  /**
   * Create a table.
   * @param {!Document} doc Document object to use to create the table.
   * @param {number} rows The number of rows in the table.  Must be >= 1.
   * @param {number} columns The number of columns in the table.  Must be >= 1.
   * @param {boolean} fillWithNbsp If true, fills table entries with
   *     {@code goog.string.Unicode.NBSP} characters.
   * @return {!HTMLTableElement} The created table.
   * @private
   */
  goog.dom.createTable_ = function(doc, rows, columns, fillWithNbsp) {
    var table = /** @type {!HTMLTableElement} */
        (doc.createElement(goog.dom.TagName.TABLE));
    var tbody = table.appendChild(doc.createElement(goog.dom.TagName.TBODY));
    for (var i = 0; i < rows; i++) {
      var tr = doc.createElement(goog.dom.TagName.TR);
      for (var j = 0; j < columns; j++) {
        var td = doc.createElement(goog.dom.TagName.TD);
        // IE <= 9 will create a text node if we set text content to the empty
        // string, so we avoid doing it unless necessary. This ensures that the
        // same DOM tree is returned on all browsers.
        if (fillWithNbsp) {
          goog.dom.setTextContent(td, goog.string.Unicode.NBSP);
        }
        tr.appendChild(td);
      }
      tbody.appendChild(tr);
    }
    return table;
  };
  
  
  
  /**
   * Creates a new Node from constant strings of HTML markup.
   * @param {...!goog.string.Const} var_args The HTML strings to concatenate then
   *     convert into a node.
   * @return {!Node}
   */
  goog.dom.constHtmlToNode = function(var_args) {
    var stringArray = goog.array.map(arguments, goog.string.Const.unwrap);
    var safeHtml =
        goog.html.uncheckedconversions
            .safeHtmlFromStringKnownToSatisfyTypeContract(
                goog.string.Const.from(
                    'Constant HTML string, that gets turned into a ' +
                    'Node later, so it will be automatically balanced.'),
                stringArray.join(''));
    return goog.dom.safeHtmlToNode(safeHtml);
  };
  
  
  /**
   * Converts HTML markup into a node. This is a safe version of
   * {@code goog.dom.htmlToDocumentFragment} which is now deleted.
   * @param {!goog.html.SafeHtml} html The HTML markup to convert.
   * @return {!Node} The resulting node.
   */
  goog.dom.safeHtmlToNode = function(html) {
    return goog.dom.safeHtmlToNode_(document, html);
  };
  
  
  /**
   * Helper for {@code safeHtmlToNode}.
   * @param {!Document} doc The document.
   * @param {!goog.html.SafeHtml} html The HTML markup to convert.
   * @return {!Node} The resulting node.
   * @private
   */
  goog.dom.safeHtmlToNode_ = function(doc, html) {
    var tempDiv = doc.createElement(goog.dom.TagName.DIV);
    if (goog.dom.BrowserFeature.INNER_HTML_NEEDS_SCOPED_ELEMENT) {
      goog.dom.safe.setInnerHtml(
          tempDiv, goog.html.SafeHtml.concat(goog.html.SafeHtml.BR, html));
      tempDiv.removeChild(tempDiv.firstChild);
    } else {
      goog.dom.safe.setInnerHtml(tempDiv, html);
    }
    return goog.dom.childrenToNode_(doc, tempDiv);
  };
  
  
  /**
   * Helper for {@code safeHtmlToNode_}.
   * @param {!Document} doc The document.
   * @param {!Node} tempDiv The input node.
   * @return {!Node} The resulting node.
   * @private
   */
  goog.dom.childrenToNode_ = function(doc, tempDiv) {
    if (tempDiv.childNodes.length == 1) {
      return tempDiv.removeChild(tempDiv.firstChild);
    } else {
      var fragment = doc.createDocumentFragment();
      while (tempDiv.firstChild) {
        fragment.appendChild(tempDiv.firstChild);
      }
      return fragment;
    }
  };
  
  
  /**
   * Returns true if the browser is in "CSS1-compatible" (standards-compliant)
   * mode, false otherwise.
   * @return {boolean} True if in CSS1-compatible mode.
   */
  goog.dom.isCss1CompatMode = function() {
    return goog.dom.isCss1CompatMode_(document);
  };
  
  
  /**
   * Returns true if the browser is in "CSS1-compatible" (standards-compliant)
   * mode, false otherwise.
   * @param {!Document} doc The document to check.
   * @return {boolean} True if in CSS1-compatible mode.
   * @private
   */
  goog.dom.isCss1CompatMode_ = function(doc) {
    if (goog.dom.COMPAT_MODE_KNOWN_) {
      return goog.dom.ASSUME_STANDARDS_MODE;
    }
  
    return doc.compatMode == 'CSS1Compat';
  };
  
  
  /**
   * Determines if the given node can contain children, intended to be used for
   * HTML generation.
   *
   * IE natively supports node.canHaveChildren but has inconsistent behavior.
   * Prior to IE8 the base tag allows children and in IE9 all nodes return true
   * for canHaveChildren.
   *
   * In practice all non-IE browsers allow you to add children to any node, but
   * the behavior is inconsistent:
   *
   * <pre>
   *   var a = goog.dom.createElement(goog.dom.TagName.BR);
   *   a.appendChild(document.createTextNode('foo'));
   *   a.appendChild(document.createTextNode('bar'));
   *   console.log(a.childNodes.length);  // 2
   *   console.log(a.innerHTML);  // Chrome: "", IE9: "foobar", FF3.5: "foobar"
   * </pre>
   *
   * For more information, see:
   * http://dev.w3.org/html5/markup/syntax.html#syntax-elements
   *
   * TODO(user): Rename shouldAllowChildren() ?
   *
   * @param {Node} node The node to check.
   * @return {boolean} Whether the node can contain children.
   */
  goog.dom.canHaveChildren = function(node) {
    if (node.nodeType != goog.dom.NodeType.ELEMENT) {
      return false;
    }
    switch (/** @type {!Element} */ (node).tagName) {
      case String(goog.dom.TagName.APPLET):
      case String(goog.dom.TagName.AREA):
      case String(goog.dom.TagName.BASE):
      case String(goog.dom.TagName.BR):
      case String(goog.dom.TagName.COL):
      case String(goog.dom.TagName.COMMAND):
      case String(goog.dom.TagName.EMBED):
      case String(goog.dom.TagName.FRAME):
      case String(goog.dom.TagName.HR):
      case String(goog.dom.TagName.IMG):
      case String(goog.dom.TagName.INPUT):
      case String(goog.dom.TagName.IFRAME):
      case String(goog.dom.TagName.ISINDEX):
      case String(goog.dom.TagName.KEYGEN):
      case String(goog.dom.TagName.LINK):
      case String(goog.dom.TagName.NOFRAMES):
      case String(goog.dom.TagName.NOSCRIPT):
      case String(goog.dom.TagName.META):
      case String(goog.dom.TagName.OBJECT):
      case String(goog.dom.TagName.PARAM):
      case String(goog.dom.TagName.SCRIPT):
      case String(goog.dom.TagName.SOURCE):
      case String(goog.dom.TagName.STYLE):
      case String(goog.dom.TagName.TRACK):
      case String(goog.dom.TagName.WBR):
        return false;
    }
    return true;
  };
  
  
  /**
   * Appends a child to a node.
   * @param {Node} parent Parent.
   * @param {Node} child Child.
   */
  goog.dom.appendChild = function(parent, child) {
    parent.appendChild(child);
  };
  
  
  /**
   * Appends a node with text or other nodes.
   * @param {!Node} parent The node to append nodes to.
   * @param {...goog.dom.Appendable} var_args The things to append to the node.
   *     If this is a Node it is appended as is.
   *     If this is a string then a text node is appended.
   *     If this is an array like object then fields 0 to length - 1 are appended.
   */
  goog.dom.append = function(parent, var_args) {
    goog.dom.append_(goog.dom.getOwnerDocument(parent), parent, arguments, 1);
  };
  
  
  /**
   * Removes all the child nodes on a DOM node.
   * @param {Node} node Node to remove children from.
   */
  goog.dom.removeChildren = function(node) {
    // Note: Iterations over live collections can be slow, this is the fastest
    // we could find. The double parenthesis are used to prevent JsCompiler and
    // strict warnings.
    var child;
    while ((child = node.firstChild)) {
      node.removeChild(child);
    }
  };
  
  
  /**
   * Inserts a new node before an existing reference node (i.e. as the previous
   * sibling). If the reference node has no parent, then does nothing.
   * @param {Node} newNode Node to insert.
   * @param {Node} refNode Reference node to insert before.
   */
  goog.dom.insertSiblingBefore = function(newNode, refNode) {
    if (refNode.parentNode) {
      refNode.parentNode.insertBefore(newNode, refNode);
    }
  };
  
  
  /**
   * Inserts a new node after an existing reference node (i.e. as the next
   * sibling). If the reference node has no parent, then does nothing.
   * @param {Node} newNode Node to insert.
   * @param {Node} refNode Reference node to insert after.
   */
  goog.dom.insertSiblingAfter = function(newNode, refNode) {
    if (refNode.parentNode) {
      refNode.parentNode.insertBefore(newNode, refNode.nextSibling);
    }
  };
  
  
  /**
   * Insert a child at a given index. If index is larger than the number of child
   * nodes that the parent currently has, the node is inserted as the last child
   * node.
   * @param {Element} parent The element into which to insert the child.
   * @param {Node} child The element to insert.
   * @param {number} index The index at which to insert the new child node. Must
   *     not be negative.
   */
  goog.dom.insertChildAt = function(parent, child, index) {
    // Note that if the second argument is null, insertBefore
    // will append the child at the end of the list of children.
    parent.insertBefore(child, parent.childNodes[index] || null);
  };
  
  
  /**
   * Removes a node from its parent.
   * @param {Node} node The node to remove.
   * @return {Node} The node removed if removed; else, null.
   */
  goog.dom.removeNode = function(node) {
    return node && node.parentNode ? node.parentNode.removeChild(node) : null;
  };
  
  
  /**
   * Replaces a node in the DOM tree. Will do nothing if {@code oldNode} has no
   * parent.
   * @param {Node} newNode Node to insert.
   * @param {Node} oldNode Node to replace.
   */
  goog.dom.replaceNode = function(newNode, oldNode) {
    var parent = oldNode.parentNode;
    if (parent) {
      parent.replaceChild(newNode, oldNode);
    }
  };
  
  
  /**
   * Flattens an element. That is, removes it and replace it with its children.
   * Does nothing if the element is not in the document.
   * @param {Element} element The element to flatten.
   * @return {Element|undefined} The original element, detached from the document
   *     tree, sans children; or undefined, if the element was not in the document
   *     to begin with.
   */
  goog.dom.flattenElement = function(element) {
    var child, parent = element.parentNode;
    if (parent && parent.nodeType != goog.dom.NodeType.DOCUMENT_FRAGMENT) {
      // Use IE DOM method (supported by Opera too) if available
      if (element.removeNode) {
        return /** @type {Element} */ (element.removeNode(false));
      } else {
        // Move all children of the original node up one level.
        while ((child = element.firstChild)) {
          parent.insertBefore(child, element);
        }
  
        // Detach the original element.
        return /** @type {Element} */ (goog.dom.removeNode(element));
      }
    }
  };
  
  
  /**
   * Returns an array containing just the element children of the given element.
   * @param {Element} element The element whose element children we want.
   * @return {!(Array<!Element>|NodeList<!Element>)} An array or array-like list
   *     of just the element children of the given element.
   */
  goog.dom.getChildren = function(element) {
    // We check if the children attribute is supported for child elements
    // since IE8 misuses the attribute by also including comments.
    if (goog.dom.BrowserFeature.CAN_USE_CHILDREN_ATTRIBUTE &&
        element.children != undefined) {
      return element.children;
    }
    // Fall back to manually filtering the element's child nodes.
    return goog.array.filter(element.childNodes, function(node) {
      return node.nodeType == goog.dom.NodeType.ELEMENT;
    });
  };
  
  
  /**
   * Returns the first child node that is an element.
   * @param {Node} node The node to get the first child element of.
   * @return {Element} The first child node of {@code node} that is an element.
   */
  goog.dom.getFirstElementChild = function(node) {
    if (goog.isDef(node.firstElementChild)) {
      return /** @type {!Element} */ (node).firstElementChild;
    }
    return goog.dom.getNextElementNode_(node.firstChild, true);
  };
  
  
  /**
   * Returns the last child node that is an element.
   * @param {Node} node The node to get the last child element of.
   * @return {Element} The last child node of {@code node} that is an element.
   */
  goog.dom.getLastElementChild = function(node) {
    if (goog.isDef(node.lastElementChild)) {
      return /** @type {!Element} */ (node).lastElementChild;
    }
    return goog.dom.getNextElementNode_(node.lastChild, false);
  };
  
  
  /**
   * Returns the first next sibling that is an element.
   * @param {Node} node The node to get the next sibling element of.
   * @return {Element} The next sibling of {@code node} that is an element.
   */
  goog.dom.getNextElementSibling = function(node) {
    if (goog.isDef(node.nextElementSibling)) {
      return /** @type {!Element} */ (node).nextElementSibling;
    }
    return goog.dom.getNextElementNode_(node.nextSibling, true);
  };
  
  
  /**
   * Returns the first previous sibling that is an element.
   * @param {Node} node The node to get the previous sibling element of.
   * @return {Element} The first previous sibling of {@code node} that is
   *     an element.
   */
  goog.dom.getPreviousElementSibling = function(node) {
    if (goog.isDef(node.previousElementSibling)) {
      return /** @type {!Element} */ (node).previousElementSibling;
    }
    return goog.dom.getNextElementNode_(node.previousSibling, false);
  };
  
  
  /**
   * Returns the first node that is an element in the specified direction,
   * starting with {@code node}.
   * @param {Node} node The node to get the next element from.
   * @param {boolean} forward Whether to look forwards or backwards.
   * @return {Element} The first element.
   * @private
   */
  goog.dom.getNextElementNode_ = function(node, forward) {
    while (node && node.nodeType != goog.dom.NodeType.ELEMENT) {
      node = forward ? node.nextSibling : node.previousSibling;
    }
  
    return /** @type {Element} */ (node);
  };
  
  
  /**
   * Returns the next node in source order from the given node.
   * @param {Node} node The node.
   * @return {Node} The next node in the DOM tree, or null if this was the last
   *     node.
   */
  goog.dom.getNextNode = function(node) {
    if (!node) {
      return null;
    }
  
    if (node.firstChild) {
      return node.firstChild;
    }
  
    while (node && !node.nextSibling) {
      node = node.parentNode;
    }
  
    return node ? node.nextSibling : null;
  };
  
  
  /**
   * Returns the previous node in source order from the given node.
   * @param {Node} node The node.
   * @return {Node} The previous node in the DOM tree, or null if this was the
   *     first node.
   */
  goog.dom.getPreviousNode = function(node) {
    if (!node) {
      return null;
    }
  
    if (!node.previousSibling) {
      return node.parentNode;
    }
  
    node = node.previousSibling;
    while (node && node.lastChild) {
      node = node.lastChild;
    }
  
    return node;
  };
  
  
  /**
   * Whether the object looks like a DOM node.
   * @param {?} obj The object being tested for node likeness.
   * @return {boolean} Whether the object looks like a DOM node.
   */
  goog.dom.isNodeLike = function(obj) {
    return goog.isObject(obj) && obj.nodeType > 0;
  };
  
  
  /**
   * Whether the object looks like an Element.
   * @param {?} obj The object being tested for Element likeness.
   * @return {boolean} Whether the object looks like an Element.
   */
  goog.dom.isElement = function(obj) {
    return goog.isObject(obj) && obj.nodeType == goog.dom.NodeType.ELEMENT;
  };
  
  
  /**
   * Returns true if the specified value is a Window object. This includes the
   * global window for HTML pages, and iframe windows.
   * @param {?} obj Variable to test.
   * @return {boolean} Whether the variable is a window.
   */
  goog.dom.isWindow = function(obj) {
    return goog.isObject(obj) && obj['window'] == obj;
  };
  
  
  /**
   * Returns an element's parent, if it's an Element.
   * @param {Element} element The DOM element.
   * @return {Element} The parent, or null if not an Element.
   */
  goog.dom.getParentElement = function(element) {
    var parent;
    if (goog.dom.BrowserFeature.CAN_USE_PARENT_ELEMENT_PROPERTY) {
      var isIe9 = goog.userAgent.IE && goog.userAgent.isVersionOrHigher('9') &&
          !goog.userAgent.isVersionOrHigher('10');
      // SVG elements in IE9 can't use the parentElement property.
      // goog.global['SVGElement'] is not defined in IE9 quirks mode.
      if (!(isIe9 && goog.global['SVGElement'] &&
            element instanceof goog.global['SVGElement'])) {
        parent = element.parentElement;
        if (parent) {
          return parent;
        }
      }
    }
    parent = element.parentNode;
    return goog.dom.isElement(parent) ? /** @type {!Element} */ (parent) : null;
  };
  
  
  /**
   * Whether a node contains another node.
   * @param {?Node} parent The node that should contain the other node.
   * @param {?Node} descendant The node to test presence of.
   * @return {boolean} Whether the parent node contains the descendent node.
   */
  goog.dom.contains = function(parent, descendant) {
    if (!parent || !descendant) {
      return false;
    }
    // We use browser specific methods for this if available since it is faster
    // that way.
  
    // IE DOM
    if (parent.contains && descendant.nodeType == goog.dom.NodeType.ELEMENT) {
      return parent == descendant || parent.contains(descendant);
    }
  
    // W3C DOM Level 3
    if (typeof parent.compareDocumentPosition != 'undefined') {
      return parent == descendant ||
          Boolean(parent.compareDocumentPosition(descendant) & 16);
    }
  
    // W3C DOM Level 1
    while (descendant && parent != descendant) {
      descendant = descendant.parentNode;
    }
    return descendant == parent;
  };
  
  
  /**
   * Compares the document order of two nodes, returning 0 if they are the same
   * node, a negative number if node1 is before node2, and a positive number if
   * node2 is before node1.  Note that we compare the order the tags appear in the
   * document so in the tree <b><i>text</i></b> the B node is considered to be
   * before the I node.
   *
   * @param {Node} node1 The first node to compare.
   * @param {Node} node2 The second node to compare.
   * @return {number} 0 if the nodes are the same node, a negative number if node1
   *     is before node2, and a positive number if node2 is before node1.
   */
  goog.dom.compareNodeOrder = function(node1, node2) {
    // Fall out quickly for equality.
    if (node1 == node2) {
      return 0;
    }
  
    // Use compareDocumentPosition where available
    if (node1.compareDocumentPosition) {
      // 4 is the bitmask for FOLLOWS.
      return node1.compareDocumentPosition(node2) & 2 ? 1 : -1;
    }
  
    // Special case for document nodes on IE 7 and 8.
    if (goog.userAgent.IE && !goog.userAgent.isDocumentModeOrHigher(9)) {
      if (node1.nodeType == goog.dom.NodeType.DOCUMENT) {
        return -1;
      }
      if (node2.nodeType == goog.dom.NodeType.DOCUMENT) {
        return 1;
      }
    }
  
    // Process in IE using sourceIndex - we check to see if the first node has
    // a source index or if its parent has one.
    if ('sourceIndex' in node1 ||
        (node1.parentNode && 'sourceIndex' in node1.parentNode)) {
      var isElement1 = node1.nodeType == goog.dom.NodeType.ELEMENT;
      var isElement2 = node2.nodeType == goog.dom.NodeType.ELEMENT;
  
      if (isElement1 && isElement2) {
        return node1.sourceIndex - node2.sourceIndex;
      } else {
        var parent1 = node1.parentNode;
        var parent2 = node2.parentNode;
  
        if (parent1 == parent2) {
          return goog.dom.compareSiblingOrder_(node1, node2);
        }
  
        if (!isElement1 && goog.dom.contains(parent1, node2)) {
          return -1 * goog.dom.compareParentsDescendantNodeIe_(node1, node2);
        }
  
  
        if (!isElement2 && goog.dom.contains(parent2, node1)) {
          return goog.dom.compareParentsDescendantNodeIe_(node2, node1);
        }
  
        return (isElement1 ? node1.sourceIndex : parent1.sourceIndex) -
            (isElement2 ? node2.sourceIndex : parent2.sourceIndex);
      }
    }
  
    // For Safari, we compare ranges.
    var doc = goog.dom.getOwnerDocument(node1);
  
    var range1, range2;
    range1 = doc.createRange();
    range1.selectNode(node1);
    range1.collapse(true);
  
    range2 = doc.createRange();
    range2.selectNode(node2);
    range2.collapse(true);
  
    return range1.compareBoundaryPoints(
        goog.global['Range'].START_TO_END, range2);
  };
  
  
  /**
   * Utility function to compare the position of two nodes, when
   * {@code textNode}'s parent is an ancestor of {@code node}.  If this entry
   * condition is not met, this function will attempt to reference a null object.
   * @param {!Node} textNode The textNode to compare.
   * @param {Node} node The node to compare.
   * @return {number} -1 if node is before textNode, +1 otherwise.
   * @private
   */
  goog.dom.compareParentsDescendantNodeIe_ = function(textNode, node) {
    var parent = textNode.parentNode;
    if (parent == node) {
      // If textNode is a child of node, then node comes first.
      return -1;
    }
    var sibling = node;
    while (sibling.parentNode != parent) {
      sibling = sibling.parentNode;
    }
    return goog.dom.compareSiblingOrder_(sibling, textNode);
  };
  
  
  /**
   * Utility function to compare the position of two nodes known to be non-equal
   * siblings.
   * @param {Node} node1 The first node to compare.
   * @param {!Node} node2 The second node to compare.
   * @return {number} -1 if node1 is before node2, +1 otherwise.
   * @private
   */
  goog.dom.compareSiblingOrder_ = function(node1, node2) {
    var s = node2;
    while ((s = s.previousSibling)) {
      if (s == node1) {
        // We just found node1 before node2.
        return -1;
      }
    }
  
    // Since we didn't find it, node1 must be after node2.
    return 1;
  };
  
  
  /**
   * Find the deepest common ancestor of the given nodes.
   * @param {...Node} var_args The nodes to find a common ancestor of.
   * @return {Node} The common ancestor of the nodes, or null if there is none.
   *     null will only be returned if two or more of the nodes are from different
   *     documents.
   */
  goog.dom.findCommonAncestor = function(var_args) {
    var i, count = arguments.length;
    if (!count) {
      return null;
    } else if (count == 1) {
      return arguments[0];
    }
  
    var paths = [];
    var minLength = Infinity;
    for (i = 0; i < count; i++) {
      // Compute the list of ancestors.
      var ancestors = [];
      var node = arguments[i];
      while (node) {
        ancestors.unshift(node);
        node = node.parentNode;
      }
  
      // Save the list for comparison.
      paths.push(ancestors);
      minLength = Math.min(minLength, ancestors.length);
    }
    var output = null;
    for (i = 0; i < minLength; i++) {
      var first = paths[0][i];
      for (var j = 1; j < count; j++) {
        if (first != paths[j][i]) {
          return output;
        }
      }
      output = first;
    }
    return output;
  };
  
  
  /**
   * Returns the owner document for a node.
   * @param {Node|Window} node The node to get the document for.
   * @return {!Document} The document owning the node.
   */
  goog.dom.getOwnerDocument = function(node) {
    // TODO(nnaze): Update param signature to be non-nullable.
    goog.asserts.assert(node, 'Node cannot be null or undefined.');
    return /** @type {!Document} */ (
        node.nodeType == goog.dom.NodeType.DOCUMENT ? node : node.ownerDocument ||
                node.document);
  };
  
  
  /**
   * Cross-browser function for getting the document element of a frame or iframe.
   * @param {Element} frame Frame element.
   * @return {!Document} The frame content document.
   */
  goog.dom.getFrameContentDocument = function(frame) {
    return frame.contentDocument ||
        /** @type {!HTMLFrameElement} */ (frame).contentWindow.document;
  };
  
  
  /**
   * Cross-browser function for getting the window of a frame or iframe.
   * @param {Element} frame Frame element.
   * @return {Window} The window associated with the given frame, or null if none
   *     exists.
   */
  goog.dom.getFrameContentWindow = function(frame) {
    try {
      return frame.contentWindow ||
          (frame.contentDocument ? goog.dom.getWindow(frame.contentDocument) :
                                   null);
    } catch (e) {
      // NOTE(user): In IE8, checking the contentWindow or contentDocument
      // properties will throw a "Unspecified Error" exception if the iframe is
      // not inserted in the DOM. If we get this we can be sure that no window
      // exists, so return null.
    }
    return null;
  };
  
  
  /**
   * Sets the text content of a node, with cross-browser support.
   * @param {Node} node The node to change the text content of.
   * @param {string|number} text The value that should replace the node's content.
   */
  goog.dom.setTextContent = function(node, text) {
    goog.asserts.assert(
        node != null,
        'goog.dom.setTextContent expects a non-null value for node');
  
    if ('textContent' in node) {
      node.textContent = text;
    } else if (node.nodeType == goog.dom.NodeType.TEXT) {
      node.data = text;
    } else if (
        node.firstChild && node.firstChild.nodeType == goog.dom.NodeType.TEXT) {
      // If the first child is a text node we just change its data and remove the
      // rest of the children.
      while (node.lastChild != node.firstChild) {
        node.removeChild(node.lastChild);
      }
      node.firstChild.data = text;
    } else {
      goog.dom.removeChildren(node);
      var doc = goog.dom.getOwnerDocument(node);
      node.appendChild(doc.createTextNode(String(text)));
    }
  };
  
  
  /**
   * Gets the outerHTML of a node, which islike innerHTML, except that it
   * actually contains the HTML of the node itself.
   * @param {Element} element The element to get the HTML of.
   * @return {string} The outerHTML of the given element.
   */
  goog.dom.getOuterHtml = function(element) {
    goog.asserts.assert(
        element !== null,
        'goog.dom.getOuterHtml expects a non-null value for element');
    // IE, Opera and WebKit all have outerHTML.
    if ('outerHTML' in element) {
      return element.outerHTML;
    } else {
      var doc = goog.dom.getOwnerDocument(element);
      var div = doc.createElement(goog.dom.TagName.DIV);
      div.appendChild(element.cloneNode(true));
      return div.innerHTML;
    }
  };
  
  
  /**
   * Finds the first descendant node that matches the filter function, using
   * a depth first search. This function offers the most general purpose way
   * of finding a matching element. You may also wish to consider
   * {@code goog.dom.query} which can express many matching criteria using
   * CSS selector expressions. These expressions often result in a more
   * compact representation of the desired result.
   * @see goog.dom.query
   *
   * @param {Node} root The root of the tree to search.
   * @param {function(Node) : boolean} p The filter function.
   * @return {Node|undefined} The found node or undefined if none is found.
   */
  goog.dom.findNode = function(root, p) {
    var rv = [];
    var found = goog.dom.findNodes_(root, p, rv, true);
    return found ? rv[0] : undefined;
  };
  
  
  /**
   * Finds all the descendant nodes that match the filter function, using a
   * a depth first search. This function offers the most general-purpose way
   * of finding a set of matching elements. You may also wish to consider
   * {@code goog.dom.query} which can express many matching criteria using
   * CSS selector expressions. These expressions often result in a more
   * compact representation of the desired result.
  
   * @param {Node} root The root of the tree to search.
   * @param {function(Node) : boolean} p The filter function.
   * @return {!Array<!Node>} The found nodes or an empty array if none are found.
   */
  goog.dom.findNodes = function(root, p) {
    var rv = [];
    goog.dom.findNodes_(root, p, rv, false);
    return rv;
  };
  
  
  /**
   * Finds the first or all the descendant nodes that match the filter function,
   * using a depth first search.
   * @param {Node} root The root of the tree to search.
   * @param {function(Node) : boolean} p The filter function.
   * @param {!Array<!Node>} rv The found nodes are added to this array.
   * @param {boolean} findOne If true we exit after the first found node.
   * @return {boolean} Whether the search is complete or not. True in case findOne
   *     is true and the node is found. False otherwise.
   * @private
   */
  goog.dom.findNodes_ = function(root, p, rv, findOne) {
    if (root != null) {
      var child = root.firstChild;
      while (child) {
        if (p(child)) {
          rv.push(child);
          if (findOne) {
            return true;
          }
        }
        if (goog.dom.findNodes_(child, p, rv, findOne)) {
          return true;
        }
        child = child.nextSibling;
      }
    }
    return false;
  };
  
  
  /**
   * Map of tags whose content to ignore when calculating text length.
   * @private {!Object<string, number>}
   * @const
   */
  goog.dom.TAGS_TO_IGNORE_ = {
    'SCRIPT': 1,
    'STYLE': 1,
    'HEAD': 1,
    'IFRAME': 1,
    'OBJECT': 1
  };
  
  
  /**
   * Map of tags which have predefined values with regard to whitespace.
   * @private {!Object<string, string>}
   * @const
   */
  goog.dom.PREDEFINED_TAG_VALUES_ = {
    'IMG': ' ',
    'BR': '\n'
  };
  
  
  /**
   * Returns true if the element has a tab index that allows it to receive
   * keyboard focus (tabIndex >= 0), false otherwise.  Note that some elements
   * natively support keyboard focus, even if they have no tab index.
   * @param {!Element} element Element to check.
   * @return {boolean} Whether the element has a tab index that allows keyboard
   *     focus.
   */
  goog.dom.isFocusableTabIndex = function(element) {
    return goog.dom.hasSpecifiedTabIndex_(element) &&
        goog.dom.isTabIndexFocusable_(element);
  };
  
  
  /**
   * Enables or disables keyboard focus support on the element via its tab index.
   * Only elements for which {@link goog.dom.isFocusableTabIndex} returns true
   * (or elements that natively support keyboard focus, like form elements) can
   * receive keyboard focus.  See http://go/tabindex for more info.
   * @param {Element} element Element whose tab index is to be changed.
   * @param {boolean} enable Whether to set or remove a tab index on the element
   *     that supports keyboard focus.
   */
  goog.dom.setFocusableTabIndex = function(element, enable) {
    if (enable) {
      element.tabIndex = 0;
    } else {
      // Set tabIndex to -1 first, then remove it. This is a workaround for
      // Safari (confirmed in version 4 on Windows). When removing the attribute
      // without setting it to -1 first, the element remains keyboard focusable
      // despite not having a tabIndex attribute anymore.
      element.tabIndex = -1;
      element.removeAttribute('tabIndex');  // Must be camelCase!
    }
  };
  
  
  /**
   * Returns true if the element can be focused, i.e. it has a tab index that
   * allows it to receive keyboard focus (tabIndex >= 0), or it is an element
   * that natively supports keyboard focus.
   * @param {!Element} element Element to check.
   * @return {boolean} Whether the element allows keyboard focus.
   */
  goog.dom.isFocusable = function(element) {
    var focusable;
    // Some elements can have unspecified tab index and still receive focus.
    if (goog.dom.nativelySupportsFocus_(element)) {
      // Make sure the element is not disabled ...
      focusable = !element.disabled &&
          // ... and if a tab index is specified, it allows focus.
          (!goog.dom.hasSpecifiedTabIndex_(element) ||
           goog.dom.isTabIndexFocusable_(element));
    } else {
      focusable = goog.dom.isFocusableTabIndex(element);
    }
  
    // IE requires elements to be visible in order to focus them.
    return focusable && goog.userAgent.IE ?
        goog.dom.hasNonZeroBoundingRect_(/** @type {!HTMLElement} */ (element)) :
        focusable;
  };
  
  
  /**
   * Returns true if the element has a specified tab index.
   * @param {!Element} element Element to check.
   * @return {boolean} Whether the element has a specified tab index.
   * @private
   */
  goog.dom.hasSpecifiedTabIndex_ = function(element) {
    // IE returns 0 for an unset tabIndex, so we must use getAttributeNode(),
    // which returns an object with a 'specified' property if tabIndex is
    // specified.  This works on other browsers, too.
    var attrNode = element.getAttributeNode('tabindex');  // Must be lowercase!
    return goog.isDefAndNotNull(attrNode) && attrNode.specified;
  };
  
  
  /**
   * Returns true if the element's tab index allows the element to be focused.
   * @param {!Element} element Element to check.
   * @return {boolean} Whether the element's tab index allows focus.
   * @private
   */
  goog.dom.isTabIndexFocusable_ = function(element) {
    var index = /** @type {!HTMLElement} */ (element).tabIndex;
    // NOTE: IE9 puts tabIndex in 16-bit int, e.g. -2 is 65534.
    return goog.isNumber(index) && index >= 0 && index < 32768;
  };
  
  
  /**
   * Returns true if the element is focusable even when tabIndex is not set.
   * @param {!Element} element Element to check.
   * @return {boolean} Whether the element natively supports focus.
   * @private
   */
  goog.dom.nativelySupportsFocus_ = function(element) {
    return element.tagName == goog.dom.TagName.A ||
        element.tagName == goog.dom.TagName.INPUT ||
        element.tagName == goog.dom.TagName.TEXTAREA ||
        element.tagName == goog.dom.TagName.SELECT ||
        element.tagName == goog.dom.TagName.BUTTON;
  };
  
  
  /**
   * Returns true if the element has a bounding rectangle that would be visible
   * (i.e. its width and height are greater than zero).
   * @param {!HTMLElement} element Element to check.
   * @return {boolean} Whether the element has a non-zero bounding rectangle.
   * @private
   */
  goog.dom.hasNonZeroBoundingRect_ = function(element) {
    var rect;
    if (!goog.isFunction(element['getBoundingClientRect']) ||
        // In IE, getBoundingClientRect throws on detached nodes.
        (goog.userAgent.IE && element.parentElement == null)) {
      rect = {'height': element.offsetHeight, 'width': element.offsetWidth};
    } else {
      rect = element.getBoundingClientRect();
    }
    return goog.isDefAndNotNull(rect) && rect.height > 0 && rect.width > 0;
  };
  
  
  /**
   * Returns the text content of the current node, without markup and invisible
   * symbols. New lines are stripped and whitespace is collapsed,
   * such that each character would be visible.
   *
   * In browsers that support it, innerText is used.  Other browsers attempt to
   * simulate it via node traversal.  Line breaks are canonicalized in IE.
   *
   * @param {Node} node The node from which we are getting content.
   * @return {string} The text content.
   */
  goog.dom.getTextContent = function(node) {
    var textContent;
    // Note(arv): IE9, Opera, and Safari 3 support innerText but they include
    // text nodes in script tags. So we revert to use a user agent test here.
    if (goog.dom.BrowserFeature.CAN_USE_INNER_TEXT && node !== null &&
        ('innerText' in node)) {
      textContent = goog.string.canonicalizeNewlines(node.innerText);
      // Unfortunately .innerText() returns text with &shy; symbols
      // We need to filter it out and then remove duplicate whitespaces
    } else {
      var buf = [];
      goog.dom.getTextContent_(node, buf, true);
      textContent = buf.join('');
    }
  
    // Strip &shy; entities. goog.format.insertWordBreaks inserts them in Opera.
    textContent = textContent.replace(/ \xAD /g, ' ').replace(/\xAD/g, '');
    // Strip &#8203; entities. goog.format.insertWordBreaks inserts them in IE8.
    textContent = textContent.replace(/\u200B/g, '');
  
    // Skip this replacement on old browsers with working innerText, which
    // automatically turns &nbsp; into ' ' and / +/ into ' ' when reading
    // innerText.
    if (!goog.dom.BrowserFeature.CAN_USE_INNER_TEXT) {
      textContent = textContent.replace(/ +/g, ' ');
    }
    if (textContent != ' ') {
      textContent = textContent.replace(/^\s*/, '');
    }
  
    return textContent;
  };
  
  
  /**
   * Returns the text content of the current node, without markup.
   *
   * Unlike {@code getTextContent} this method does not collapse whitespaces
   * or normalize lines breaks.
   *
   * @param {Node} node The node from which we are getting content.
   * @return {string} The raw text content.
   */
  goog.dom.getRawTextContent = function(node) {
    var buf = [];
    goog.dom.getTextContent_(node, buf, false);
  
    return buf.join('');
  };
  
  
  /**
   * Recursive support function for text content retrieval.
   *
   * @param {Node} node The node from which we are getting content.
   * @param {Array<string>} buf string buffer.
   * @param {boolean} normalizeWhitespace Whether to normalize whitespace.
   * @private
   */
  goog.dom.getTextContent_ = function(node, buf, normalizeWhitespace) {
    if (node.nodeName in goog.dom.TAGS_TO_IGNORE_) {
      // ignore certain tags
    } else if (node.nodeType == goog.dom.NodeType.TEXT) {
      if (normalizeWhitespace) {
        buf.push(String(node.nodeValue).replace(/(\r\n|\r|\n)/g, ''));
      } else {
        buf.push(node.nodeValue);
      }
    } else if (node.nodeName in goog.dom.PREDEFINED_TAG_VALUES_) {
      buf.push(goog.dom.PREDEFINED_TAG_VALUES_[node.nodeName]);
    } else {
      var child = node.firstChild;
      while (child) {
        goog.dom.getTextContent_(child, buf, normalizeWhitespace);
        child = child.nextSibling;
      }
    }
  };
  
  
  /**
   * Returns the text length of the text contained in a node, without markup. This
   * is equivalent to the selection length if the node was selected, or the number
   * of cursor movements to traverse the node. Images & BRs take one space.  New
   * lines are ignored.
   *
   * @param {Node} node The node whose text content length is being calculated.
   * @return {number} The length of {@code node}'s text content.
   */
  goog.dom.getNodeTextLength = function(node) {
    return goog.dom.getTextContent(node).length;
  };
  
  
  /**
   * Returns the text offset of a node relative to one of its ancestors. The text
   * length is the same as the length calculated by goog.dom.getNodeTextLength.
   *
   * @param {Node} node The node whose offset is being calculated.
   * @param {Node=} opt_offsetParent The node relative to which the offset will
   *     be calculated. Defaults to the node's owner document's body.
   * @return {number} The text offset.
   */
  goog.dom.getNodeTextOffset = function(node, opt_offsetParent) {
    var root = opt_offsetParent || goog.dom.getOwnerDocument(node).body;
    var buf = [];
    while (node && node != root) {
      var cur = node;
      while ((cur = cur.previousSibling)) {
        buf.unshift(goog.dom.getTextContent(cur));
      }
      node = node.parentNode;
    }
    // Trim left to deal with FF cases when there might be line breaks and empty
    // nodes at the front of the text
    return goog.string.trimLeft(buf.join('')).replace(/ +/g, ' ').length;
  };
  
  
  /**
   * Returns the node at a given offset in a parent node.  If an object is
   * provided for the optional third parameter, the node and the remainder of the
   * offset will stored as properties of this object.
   * @param {Node} parent The parent node.
   * @param {number} offset The offset into the parent node.
   * @param {Object=} opt_result Object to be used to store the return value. The
   *     return value will be stored in the form {node: Node, remainder: number}
   *     if this object is provided.
   * @return {Node} The node at the given offset.
   */
  goog.dom.getNodeAtOffset = function(parent, offset, opt_result) {
    var stack = [parent], pos = 0, cur = null;
    while (stack.length > 0 && pos < offset) {
      cur = stack.pop();
      if (cur.nodeName in goog.dom.TAGS_TO_IGNORE_) {
        // ignore certain tags
      } else if (cur.nodeType == goog.dom.NodeType.TEXT) {
        var text = cur.nodeValue.replace(/(\r\n|\r|\n)/g, '').replace(/ +/g, ' ');
        pos += text.length;
      } else if (cur.nodeName in goog.dom.PREDEFINED_TAG_VALUES_) {
        pos += goog.dom.PREDEFINED_TAG_VALUES_[cur.nodeName].length;
      } else {
        for (var i = cur.childNodes.length - 1; i >= 0; i--) {
          stack.push(cur.childNodes[i]);
        }
      }
    }
    if (goog.isObject(opt_result)) {
      opt_result.remainder = cur ? cur.nodeValue.length + offset - pos - 1 : 0;
      opt_result.node = cur;
    }
  
    return cur;
  };
  
  
  /**
   * Returns true if the object is a {@code NodeList}.  To qualify as a NodeList,
   * the object must have a numeric length property and an item function (which
   * has type 'string' on IE for some reason).
   * @param {Object} val Object to test.
   * @return {boolean} Whether the object is a NodeList.
   */
  goog.dom.isNodeList = function(val) {
    // TODO(attila): Now the isNodeList is part of goog.dom we can use
    // goog.userAgent to make this simpler.
    // A NodeList must have a length property of type 'number' on all platforms.
    if (val && typeof val.length == 'number') {
      // A NodeList is an object everywhere except Safari, where it's a function.
      if (goog.isObject(val)) {
        // A NodeList must have an item function (on non-IE platforms) or an item
        // property of type 'string' (on IE).
        return typeof val.item == 'function' || typeof val.item == 'string';
      } else if (goog.isFunction(val)) {
        // On Safari, a NodeList is a function with an item property that is also
        // a function.
        return typeof val.item == 'function';
      }
    }
  
    // Not a NodeList.
    return false;
  };
  
  
  /**
   * Walks up the DOM hierarchy returning the first ancestor that has the passed
   * tag name and/or class name. If the passed element matches the specified
   * criteria, the element itself is returned.
   * @param {Node} element The DOM node to start with.
   * @param {?(goog.dom.TagName|string)=} opt_tag The tag name to match (or
   *     null/undefined to match only based on class name).
   * @param {?string=} opt_class The class name to match (or null/undefined to
   *     match only based on tag name).
   * @param {number=} opt_maxSearchSteps Maximum number of levels to search up the
   *     dom.
   * @return {Element} The first ancestor that matches the passed criteria, or
   *     null if no match is found.
   */
  goog.dom.getAncestorByTagNameAndClass = function(
      element, opt_tag, opt_class, opt_maxSearchSteps) {
    if (!opt_tag && !opt_class) {
      return null;
    }
    var tagName = opt_tag ? opt_tag.toUpperCase() : null;
    return /** @type {Element} */ (goog.dom.getAncestor(element, function(node) {
      return (!tagName || node.nodeName == tagName) &&
          (!opt_class ||
           goog.isString(node.className) &&
               goog.array.contains(node.className.split(/\s+/), opt_class));
    }, true, opt_maxSearchSteps));
  };
  
  
  /**
   * Walks up the DOM hierarchy returning the first ancestor that has the passed
   * class name. If the passed element matches the specified criteria, the
   * element itself is returned.
   * @param {Node} element The DOM node to start with.
   * @param {string} className The class name to match.
   * @param {number=} opt_maxSearchSteps Maximum number of levels to search up the
   *     dom.
   * @return {Element} The first ancestor that matches the passed criteria, or
   *     null if none match.
   */
  goog.dom.getAncestorByClass = function(element, className, opt_maxSearchSteps) {
    return goog.dom.getAncestorByTagNameAndClass(
        element, null, className, opt_maxSearchSteps);
  };
  
  
  /**
   * Walks up the DOM hierarchy returning the first ancestor that passes the
   * matcher function.
   * @param {Node} element The DOM node to start with.
   * @param {function(Node) : boolean} matcher A function that returns true if the
   *     passed node matches the desired criteria.
   * @param {boolean=} opt_includeNode If true, the node itself is included in
   *     the search (the first call to the matcher will pass startElement as
   *     the node to test).
   * @param {number=} opt_maxSearchSteps Maximum number of levels to search up the
   *     dom.
   * @return {Node} DOM node that matched the matcher, or null if there was
   *     no match.
   */
  goog.dom.getAncestor = function(
      element, matcher, opt_includeNode, opt_maxSearchSteps) {
    if (!opt_includeNode) {
      element = element.parentNode;
    }
    var steps = 0;
    while (element &&
           (opt_maxSearchSteps == null || steps <= opt_maxSearchSteps)) {
      goog.asserts.assert(element.name != 'parentNode');
      if (matcher(element)) {
        return element;
      }
      element = element.parentNode;
      steps++;
    }
    // Reached the root of the DOM without a match
    return null;
  };
  
  
  /**
   * Determines the active element in the given document.
   * @param {Document} doc The document to look in.
   * @return {Element} The active element.
   */
  goog.dom.getActiveElement = function(doc) {
    try {
      return doc && doc.activeElement;
    } catch (e) {
      // NOTE(nicksantos): Sometimes, evaluating document.activeElement in IE
      // throws an exception. I'm not 100% sure why, but I suspect it chokes
      // on document.activeElement if the activeElement has been recently
      // removed from the DOM by a JS operation.
      //
      // We assume that an exception here simply means
      // "there is no active element."
    }
  
    return null;
  };
  
  
  /**
   * Gives the current devicePixelRatio.
   *
   * By default, this is the value of window.devicePixelRatio (which should be
   * preferred if present).
   *
   * If window.devicePixelRatio is not present, the ratio is calculated with
   * window.matchMedia, if present. Otherwise, gives 1.0.
   *
   * Some browsers (including Chrome) consider the browser zoom level in the pixel
   * ratio, so the value may change across multiple calls.
   *
   * @return {number} The number of actual pixels per virtual pixel.
   */
  goog.dom.getPixelRatio = function() {
    var win = goog.dom.getWindow();
    if (goog.isDef(win.devicePixelRatio)) {
      return win.devicePixelRatio;
    } else if (win.matchMedia) {
      return goog.dom.matchesPixelRatio_(.75) ||
          goog.dom.matchesPixelRatio_(1.5) || goog.dom.matchesPixelRatio_(2) ||
          goog.dom.matchesPixelRatio_(3) || 1;
    }
    return 1;
  };
  
  
  /**
   * Calculates a mediaQuery to check if the current device supports the
   * given actual to virtual pixel ratio.
   * @param {number} pixelRatio The ratio of actual pixels to virtual pixels.
   * @return {number} pixelRatio if applicable, otherwise 0.
   * @private
   */
  goog.dom.matchesPixelRatio_ = function(pixelRatio) {
    var win = goog.dom.getWindow();
    var query =
        ('(-webkit-min-device-pixel-ratio: ' + pixelRatio + '),' +
         '(min--moz-device-pixel-ratio: ' + pixelRatio + '),' +
         '(min-resolution: ' + pixelRatio + 'dppx)');
    return win.matchMedia(query).matches ? pixelRatio : 0;
  };
  
  
  /**
   * Gets '2d' context of a canvas. Shortcut for canvas.getContext('2d') with a
   * type information.
   * @param {!HTMLCanvasElement} canvas
   * @return {!CanvasRenderingContext2D}
   */
  goog.dom.getCanvasContext2D = function(canvas) {
    return /** @type {!CanvasRenderingContext2D} */ (canvas.getContext('2d'));
  };
  
  
  
  /**
   * Create an instance of a DOM helper with a new document object.
   * @param {Document=} opt_document Document object to associate with this
   *     DOM helper.
   * @constructor
   */
  goog.dom.DomHelper = function(opt_document) {
    /**
     * Reference to the document object to use
     * @type {!Document}
     * @private
     */
    this.document_ = opt_document || goog.global.document || document;
  };
  
  
  /**
   * Gets the dom helper object for the document where the element resides.
   * @param {Node=} opt_node If present, gets the DomHelper for this node.
   * @return {!goog.dom.DomHelper} The DomHelper.
   */
  goog.dom.DomHelper.prototype.getDomHelper = goog.dom.getDomHelper;
  
  
  /**
   * Sets the document object.
   * @param {!Document} document Document object.
   */
  goog.dom.DomHelper.prototype.setDocument = function(document) {
    this.document_ = document;
  };
  
  
  /**
   * Gets the document object being used by the dom library.
   * @return {!Document} Document object.
   */
  goog.dom.DomHelper.prototype.getDocument = function() {
    return this.document_;
  };
  
  
  /**
   * Alias for {@code getElementById}. If a DOM node is passed in then we just
   * return that.
   * @param {string|Element} element Element ID or a DOM node.
   * @return {Element} The element with the given ID, or the node passed in.
   */
  goog.dom.DomHelper.prototype.getElement = function(element) {
    return goog.dom.getElementHelper_(this.document_, element);
  };
  
  
  /**
   * Gets an element by id, asserting that the element is found.
   *
   * This is used when an element is expected to exist, and should fail with
   * an assertion error if it does not (if assertions are enabled).
   *
   * @param {string} id Element ID.
   * @return {!Element} The element with the given ID, if it exists.
   */
  goog.dom.DomHelper.prototype.getRequiredElement = function(id) {
    return goog.dom.getRequiredElementHelper_(this.document_, id);
  };
  
  
  /**
   * Alias for {@code getElement}.
   * @param {string|Element} element Element ID or a DOM node.
   * @return {Element} The element with the given ID, or the node passed in.
   * @deprecated Use {@link goog.dom.DomHelper.prototype.getElement} instead.
   */
  goog.dom.DomHelper.prototype.$ = goog.dom.DomHelper.prototype.getElement;
  
  
  /**
   * Gets elements by tag name.
   * @param {goog.dom.TagName} tagName
   * @param {(!Document|!Element)=} opt_parent Parent element or document where to
   *     look for elements. Defaults to document of this DomHelper.
   * @return {!NodeList<!Element>}
   */
  goog.dom.DomHelper.prototype.getElementsByTagName =
      function(tagName, opt_parent) {
    var parent = opt_parent || this.document_;
    return parent.getElementsByTagName(tagName);
  };
  
  
  /**
   * Looks up elements by both tag and class name, using browser native functions
   * ({@code querySelectorAll}, {@code getElementsByTagName} or
   * {@code getElementsByClassName}) where possible. The returned array is a live
   * NodeList or a static list depending on the code path taken.
   *
   * @see goog.dom.query
   *
   * @param {?string=} opt_tag Element tag name or * for all tags.
   * @param {?string=} opt_class Optional class name.
   * @param {(Document|Element)=} opt_el Optional element to look in.
   * @return {!IArrayLike<!Element>} Array-like list of elements (only a length
   *     property and numerical indices are guaranteed to exist).
   */
  goog.dom.DomHelper.prototype.getElementsByTagNameAndClass = function(
      opt_tag, opt_class, opt_el) {
    return goog.dom.getElementsByTagNameAndClass_(
        this.document_, opt_tag, opt_class, opt_el);
  };
  
  
  /**
   * Returns an array of all the elements with the provided className.
   * @see {goog.dom.query}
   * @param {string} className the name of the class to look for.
   * @param {Element|Document=} opt_el Optional element to look in.
   * @return {!IArrayLike<!Element>} The items found with the class name provided.
   */
  goog.dom.DomHelper.prototype.getElementsByClass = function(className, opt_el) {
    var doc = opt_el || this.document_;
    return goog.dom.getElementsByClass(className, doc);
  };
  
  
  /**
   * Returns the first element we find matching the provided class name.
   * @see {goog.dom.query}
   * @param {string} className the name of the class to look for.
   * @param {(Element|Document)=} opt_el Optional element to look in.
   * @return {Element} The first item found with the class name provided.
   */
  goog.dom.DomHelper.prototype.getElementByClass = function(className, opt_el) {
    var doc = opt_el || this.document_;
    return goog.dom.getElementByClass(className, doc);
  };
  
  
  /**
   * Ensures an element with the given className exists, and then returns the
   * first element with the provided className.
   * @see {goog.dom.query}
   * @param {string} className the name of the class to look for.
   * @param {(!Element|!Document)=} opt_root Optional element or document to look
   *     in.
   * @return {!Element} The first item found with the class name provided.
   * @throws {goog.asserts.AssertionError} Thrown if no element is found.
   */
  goog.dom.DomHelper.prototype.getRequiredElementByClass = function(
      className, opt_root) {
    var root = opt_root || this.document_;
    return goog.dom.getRequiredElementByClass(className, root);
  };
  
  
  /**
   * Alias for {@code getElementsByTagNameAndClass}.
   * @deprecated Use DomHelper getElementsByTagNameAndClass.
   * @see goog.dom.query
   *
   * @param {?string=} opt_tag Element tag name.
   * @param {?string=} opt_class Optional class name.
   * @param {Element=} opt_el Optional element to look in.
   * @return {!IArrayLike<!Element>} Array-like list of elements (only a length
   *     property and numerical indices are guaranteed to exist).
   */
  goog.dom.DomHelper.prototype.$$ =
      goog.dom.DomHelper.prototype.getElementsByTagNameAndClass;
  
  
  /**
   * Sets a number of properties on a node.
   * @param {Element} element DOM node to set properties on.
   * @param {Object} properties Hash of property:value pairs.
   */
  goog.dom.DomHelper.prototype.setProperties = goog.dom.setProperties;
  
  
  /**
   * Gets the dimensions of the viewport.
   * @param {Window=} opt_window Optional window element to test. Defaults to
   *     the window of the Dom Helper.
   * @return {!goog.math.Size} Object with values 'width' and 'height'.
   */
  goog.dom.DomHelper.prototype.getViewportSize = function(opt_window) {
    // TODO(arv): This should not take an argument. That breaks the rule of a
    // a DomHelper representing a single frame/window/document.
    return goog.dom.getViewportSize(opt_window || this.getWindow());
  };
  
  
  /**
   * Calculates the height of the document.
   *
   * @return {number} The height of the document.
   */
  goog.dom.DomHelper.prototype.getDocumentHeight = function() {
    return goog.dom.getDocumentHeight_(this.getWindow());
  };
  
  
  /**
   * Typedef for use with goog.dom.createDom and goog.dom.append.
   * @typedef {Object|string|Array|NodeList}
   */
  goog.dom.Appendable;
  
  
  /**
   * Returns a dom node with a set of attributes.  This function accepts varargs
   * for subsequent nodes to be added.  Subsequent nodes will be added to the
   * first node as childNodes.
   *
   * So:
   * <code>createDom(goog.dom.TagName.DIV, null, createDom(goog.dom.TagName.P), createDom(goog.dom.TagName.P));</code>
   * would return a div with two child paragraphs
   *
   * An easy way to move all child nodes of an existing element to a new parent
   * element is:
   * <code>createDom(goog.dom.TagName.DIV, null, oldElement.childNodes);</code>
   * which will remove all child nodes from the old element and add them as
   * child nodes of the new DIV.
   *
   * @param {string} tagName Tag to create.
   * @param {Object|string=} opt_attributes If object, then a map of name-value
   *     pairs for attributes. If a string, then this is the className of the new
   *     element.
   * @param {...goog.dom.Appendable} var_args Further DOM nodes or
   *     strings for text nodes. If one of the var_args is an array or
   *     NodeList, its elements will be added as childNodes instead.
   * @return {!Element} Reference to a DOM node.
   */
  goog.dom.DomHelper.prototype.createDom = function(
      tagName, opt_attributes, var_args) {
    return goog.dom.createDom_(this.document_, arguments);
  };
  
  
  /**
   * Alias for {@code createDom}.
   * @param {string} tagName Tag to create.
   * @param {(Object|string)=} opt_attributes If object, then a map of name-value
   *     pairs for attributes. If a string, then this is the className of the new
   *     element.
   * @param {...goog.dom.Appendable} var_args Further DOM nodes or strings for
   *     text nodes.  If one of the var_args is an array, its children will be
   *     added as childNodes instead.
   * @return {!Element} Reference to a DOM node.
   * @deprecated Use {@link goog.dom.DomHelper.prototype.createDom} instead.
   */
  goog.dom.DomHelper.prototype.$dom = goog.dom.DomHelper.prototype.createDom;
  
  
  /**
   * Creates a new element.
   * @param {string} name Tag name.
   * @return {!Element} The new element.
   */
  goog.dom.DomHelper.prototype.createElement = function(name) {
    return this.document_.createElement(String(name));
  };
  
  
  /**
   * Creates a new text node.
   * @param {number|string} content Content.
   * @return {!Text} The new text node.
   */
  goog.dom.DomHelper.prototype.createTextNode = function(content) {
    return this.document_.createTextNode(String(content));
  };
  
  
  /**
   * Create a table.
   * @param {number} rows The number of rows in the table.  Must be >= 1.
   * @param {number} columns The number of columns in the table.  Must be >= 1.
   * @param {boolean=} opt_fillWithNbsp If true, fills table entries with
   *     {@code goog.string.Unicode.NBSP} characters.
   * @return {!HTMLElement} The created table.
   */
  goog.dom.DomHelper.prototype.createTable = function(
      rows, columns, opt_fillWithNbsp) {
    return goog.dom.createTable_(
        this.document_, rows, columns, !!opt_fillWithNbsp);
  };
  
  
  /**
   * Converts an HTML into a node or a document fragment. A single Node is used if
   * {@code html} only generates a single node. If {@code html} generates multiple
   * nodes then these are put inside a {@code DocumentFragment}. This is a safe
   * version of {@code goog.dom.DomHelper#htmlToDocumentFragment} which is now
   * deleted.
   * @param {!goog.html.SafeHtml} html The HTML markup to convert.
   * @return {!Node} The resulting node.
   */
  goog.dom.DomHelper.prototype.safeHtmlToNode = function(html) {
    return goog.dom.safeHtmlToNode_(this.document_, html);
  };
  
  
  /**
   * Returns true if the browser is in "CSS1-compatible" (standards-compliant)
   * mode, false otherwise.
   * @return {boolean} True if in CSS1-compatible mode.
   */
  goog.dom.DomHelper.prototype.isCss1CompatMode = function() {
    return goog.dom.isCss1CompatMode_(this.document_);
  };
  
  
  /**
   * Gets the window object associated with the document.
   * @return {!Window} The window associated with the given document.
   */
  goog.dom.DomHelper.prototype.getWindow = function() {
    return goog.dom.getWindow_(this.document_);
  };
  
  
  /**
   * Gets the document scroll element.
   * @return {!Element} Scrolling element.
   */
  goog.dom.DomHelper.prototype.getDocumentScrollElement = function() {
    return goog.dom.getDocumentScrollElement_(this.document_);
  };
  
  
  /**
   * Gets the document scroll distance as a coordinate object.
   * @return {!goog.math.Coordinate} Object with properties 'x' and 'y'.
   */
  goog.dom.DomHelper.prototype.getDocumentScroll = function() {
    return goog.dom.getDocumentScroll_(this.document_);
  };
  
  
  /**
   * Determines the active element in the given document.
   * @param {Document=} opt_doc The document to look in.
   * @return {Element} The active element.
   */
  goog.dom.DomHelper.prototype.getActiveElement = function(opt_doc) {
    return goog.dom.getActiveElement(opt_doc || this.document_);
  };
  
  
  /**
   * Appends a child to a node.
   * @param {Node} parent Parent.
   * @param {Node} child Child.
   */
  goog.dom.DomHelper.prototype.appendChild = goog.dom.appendChild;
  
  
  /**
   * Appends a node with text or other nodes.
   * @param {!Node} parent The node to append nodes to.
   * @param {...goog.dom.Appendable} var_args The things to append to the node.
   *     If this is a Node it is appended as is.
   *     If this is a string then a text node is appended.
   *     If this is an array like object then fields 0 to length - 1 are appended.
   */
  goog.dom.DomHelper.prototype.append = goog.dom.append;
  
  
  /**
   * Determines if the given node can contain children, intended to be used for
   * HTML generation.
   *
   * @param {Node} node The node to check.
   * @return {boolean} Whether the node can contain children.
   */
  goog.dom.DomHelper.prototype.canHaveChildren = goog.dom.canHaveChildren;
  
  
  /**
   * Removes all the child nodes on a DOM node.
   * @param {Node} node Node to remove children from.
   */
  goog.dom.DomHelper.prototype.removeChildren = goog.dom.removeChildren;
  
  
  /**
   * Inserts a new node before an existing reference node (i.e., as the previous
   * sibling). If the reference node has no parent, then does nothing.
   * @param {Node} newNode Node to insert.
   * @param {Node} refNode Reference node to insert before.
   */
  goog.dom.DomHelper.prototype.insertSiblingBefore = goog.dom.insertSiblingBefore;
  
  
  /**
   * Inserts a new node after an existing reference node (i.e., as the next
   * sibling). If the reference node has no parent, then does nothing.
   * @param {Node} newNode Node to insert.
   * @param {Node} refNode Reference node to insert after.
   */
  goog.dom.DomHelper.prototype.insertSiblingAfter = goog.dom.insertSiblingAfter;
  
  
  /**
   * Insert a child at a given index. If index is larger than the number of child
   * nodes that the parent currently has, the node is inserted as the last child
   * node.
   * @param {Element} parent The element into which to insert the child.
   * @param {Node} child The element to insert.
   * @param {number} index The index at which to insert the new child node. Must
   *     not be negative.
   */
  goog.dom.DomHelper.prototype.insertChildAt = goog.dom.insertChildAt;
  
  
  /**
   * Removes a node from its parent.
   * @param {Node} node The node to remove.
   * @return {Node} The node removed if removed; else, null.
   */
  goog.dom.DomHelper.prototype.removeNode = goog.dom.removeNode;
  
  
  /**
   * Replaces a node in the DOM tree. Will do nothing if {@code oldNode} has no
   * parent.
   * @param {Node} newNode Node to insert.
   * @param {Node} oldNode Node to replace.
   */
  goog.dom.DomHelper.prototype.replaceNode = goog.dom.replaceNode;
  
  
  /**
   * Flattens an element. That is, removes it and replace it with its children.
   * @param {Element} element The element to flatten.
   * @return {Element|undefined} The original element, detached from the document
   *     tree, sans children, or undefined if the element was already not in the
   *     document.
   */
  goog.dom.DomHelper.prototype.flattenElement = goog.dom.flattenElement;
  
  
  /**
   * Returns an array containing just the element children of the given element.
   * @param {Element} element The element whose element children we want.
   * @return {!(Array<!Element>|NodeList<!Element>)} An array or array-like list
   *     of just the element children of the given element.
   */
  goog.dom.DomHelper.prototype.getChildren = goog.dom.getChildren;
  
  
  /**
   * Returns the first child node that is an element.
   * @param {Node} node The node to get the first child element of.
   * @return {Element} The first child node of {@code node} that is an element.
   */
  goog.dom.DomHelper.prototype.getFirstElementChild =
      goog.dom.getFirstElementChild;
  
  
  /**
   * Returns the last child node that is an element.
   * @param {Node} node The node to get the last child element of.
   * @return {Element} The last child node of {@code node} that is an element.
   */
  goog.dom.DomHelper.prototype.getLastElementChild = goog.dom.getLastElementChild;
  
  
  /**
   * Returns the first next sibling that is an element.
   * @param {Node} node The node to get the next sibling element of.
   * @return {Element} The next sibling of {@code node} that is an element.
   */
  goog.dom.DomHelper.prototype.getNextElementSibling =
      goog.dom.getNextElementSibling;
  
  
  /**
   * Returns the first previous sibling that is an element.
   * @param {Node} node The node to get the previous sibling element of.
   * @return {Element} The first previous sibling of {@code node} that is
   *     an element.
   */
  goog.dom.DomHelper.prototype.getPreviousElementSibling =
      goog.dom.getPreviousElementSibling;
  
  
  /**
   * Returns the next node in source order from the given node.
   * @param {Node} node The node.
   * @return {Node} The next node in the DOM tree, or null if this was the last
   *     node.
   */
  goog.dom.DomHelper.prototype.getNextNode = goog.dom.getNextNode;
  
  
  /**
   * Returns the previous node in source order from the given node.
   * @param {Node} node The node.
   * @return {Node} The previous node in the DOM tree, or null if this was the
   *     first node.
   */
  goog.dom.DomHelper.prototype.getPreviousNode = goog.dom.getPreviousNode;
  
  
  /**
   * Whether the object looks like a DOM node.
   * @param {?} obj The object being tested for node likeness.
   * @return {boolean} Whether the object looks like a DOM node.
   */
  goog.dom.DomHelper.prototype.isNodeLike = goog.dom.isNodeLike;
  
  
  /**
   * Whether the object looks like an Element.
   * @param {?} obj The object being tested for Element likeness.
   * @return {boolean} Whether the object looks like an Element.
   */
  goog.dom.DomHelper.prototype.isElement = goog.dom.isElement;
  
  
  /**
   * Returns true if the specified value is a Window object. This includes the
   * global window for HTML pages, and iframe windows.
   * @param {?} obj Variable to test.
   * @return {boolean} Whether the variable is a window.
   */
  goog.dom.DomHelper.prototype.isWindow = goog.dom.isWindow;
  
  
  /**
   * Returns an element's parent, if it's an Element.
   * @param {Element} element The DOM element.
   * @return {Element} The parent, or null if not an Element.
   */
  goog.dom.DomHelper.prototype.getParentElement = goog.dom.getParentElement;
  
  
  /**
   * Whether a node contains another node.
   * @param {Node} parent The node that should contain the other node.
   * @param {Node} descendant The node to test presence of.
   * @return {boolean} Whether the parent node contains the descendent node.
   */
  goog.dom.DomHelper.prototype.contains = goog.dom.contains;
  
  
  /**
   * Compares the document order of two nodes, returning 0 if they are the same
   * node, a negative number if node1 is before node2, and a positive number if
   * node2 is before node1.  Note that we compare the order the tags appear in the
   * document so in the tree <b><i>text</i></b> the B node is considered to be
   * before the I node.
   *
   * @param {Node} node1 The first node to compare.
   * @param {Node} node2 The second node to compare.
   * @return {number} 0 if the nodes are the same node, a negative number if node1
   *     is before node2, and a positive number if node2 is before node1.
   */
  goog.dom.DomHelper.prototype.compareNodeOrder = goog.dom.compareNodeOrder;
  
  
  /**
   * Find the deepest common ancestor of the given nodes.
   * @param {...Node} var_args The nodes to find a common ancestor of.
   * @return {Node} The common ancestor of the nodes, or null if there is none.
   *     null will only be returned if two or more of the nodes are from different
   *     documents.
   */
  goog.dom.DomHelper.prototype.findCommonAncestor = goog.dom.findCommonAncestor;
  
  
  /**
   * Returns the owner document for a node.
   * @param {Node} node The node to get the document for.
   * @return {!Document} The document owning the node.
   */
  goog.dom.DomHelper.prototype.getOwnerDocument = goog.dom.getOwnerDocument;
  
  
  /**
   * Cross browser function for getting the document element of an iframe.
   * @param {Element} iframe Iframe element.
   * @return {!Document} The frame content document.
   */
  goog.dom.DomHelper.prototype.getFrameContentDocument =
      goog.dom.getFrameContentDocument;
  
  
  /**
   * Cross browser function for getting the window of a frame or iframe.
   * @param {Element} frame Frame element.
   * @return {Window} The window associated with the given frame.
   */
  goog.dom.DomHelper.prototype.getFrameContentWindow =
      goog.dom.getFrameContentWindow;
  
  
  /**
   * Sets the text content of a node, with cross-browser support.
   * @param {Node} node The node to change the text content of.
   * @param {string|number} text The value that should replace the node's content.
   */
  goog.dom.DomHelper.prototype.setTextContent = goog.dom.setTextContent;
  
  
  /**
   * Gets the outerHTML of a node, which islike innerHTML, except that it
   * actually contains the HTML of the node itself.
   * @param {Element} element The element to get the HTML of.
   * @return {string} The outerHTML of the given element.
   */
  goog.dom.DomHelper.prototype.getOuterHtml = goog.dom.getOuterHtml;
  
  
  /**
   * Finds the first descendant node that matches the filter function. This does
   * a depth first search.
   * @param {Node} root The root of the tree to search.
   * @param {function(Node) : boolean} p The filter function.
   * @return {Node|undefined} The found node or undefined if none is found.
   */
  goog.dom.DomHelper.prototype.findNode = goog.dom.findNode;
  
  
  /**
   * Finds all the descendant nodes that matches the filter function. This does a
   * depth first search.
   * @param {Node} root The root of the tree to search.
   * @param {function(Node) : boolean} p The filter function.
   * @return {Array<Node>} The found nodes or an empty array if none are found.
   */
  goog.dom.DomHelper.prototype.findNodes = goog.dom.findNodes;
  
  
  /**
   * Returns true if the element has a tab index that allows it to receive
   * keyboard focus (tabIndex >= 0), false otherwise.  Note that some elements
   * natively support keyboard focus, even if they have no tab index.
   * @param {!Element} element Element to check.
   * @return {boolean} Whether the element has a tab index that allows keyboard
   *     focus.
   */
  goog.dom.DomHelper.prototype.isFocusableTabIndex = goog.dom.isFocusableTabIndex;
  
  
  /**
   * Enables or disables keyboard focus support on the element via its tab index.
   * Only elements for which {@link goog.dom.isFocusableTabIndex} returns true
   * (or elements that natively support keyboard focus, like form elements) can
   * receive keyboard focus.  See http://go/tabindex for more info.
   * @param {Element} element Element whose tab index is to be changed.
   * @param {boolean} enable Whether to set or remove a tab index on the element
   *     that supports keyboard focus.
   */
  goog.dom.DomHelper.prototype.setFocusableTabIndex =
      goog.dom.setFocusableTabIndex;
  
  
  /**
   * Returns true if the element can be focused, i.e. it has a tab index that
   * allows it to receive keyboard focus (tabIndex >= 0), or it is an element
   * that natively supports keyboard focus.
   * @param {!Element} element Element to check.
   * @return {boolean} Whether the element allows keyboard focus.
   */
  goog.dom.DomHelper.prototype.isFocusable = goog.dom.isFocusable;
  
  
  /**
   * Returns the text contents of the current node, without markup. New lines are
   * stripped and whitespace is collapsed, such that each character would be
   * visible.
   *
   * In browsers that support it, innerText is used.  Other browsers attempt to
   * simulate it via node traversal.  Line breaks are canonicalized in IE.
   *
   * @param {Node} node The node from which we are getting content.
   * @return {string} The text content.
   */
  goog.dom.DomHelper.prototype.getTextContent = goog.dom.getTextContent;
  
  
  /**
   * Returns the text length of the text contained in a node, without markup. This
   * is equivalent to the selection length if the node was selected, or the number
   * of cursor movements to traverse the node. Images & BRs take one space.  New
   * lines are ignored.
   *
   * @param {Node} node The node whose text content length is being calculated.
   * @return {number} The length of {@code node}'s text content.
   */
  goog.dom.DomHelper.prototype.getNodeTextLength = goog.dom.getNodeTextLength;
  
  
  /**
   * Returns the text offset of a node relative to one of its ancestors. The text
   * length is the same as the length calculated by
   * {@code goog.dom.getNodeTextLength}.
   *
   * @param {Node} node The node whose offset is being calculated.
   * @param {Node=} opt_offsetParent Defaults to the node's owner document's body.
   * @return {number} The text offset.
   */
  goog.dom.DomHelper.prototype.getNodeTextOffset = goog.dom.getNodeTextOffset;
  
  
  /**
   * Returns the node at a given offset in a parent node.  If an object is
   * provided for the optional third parameter, the node and the remainder of the
   * offset will stored as properties of this object.
   * @param {Node} parent The parent node.
   * @param {number} offset The offset into the parent node.
   * @param {Object=} opt_result Object to be used to store the return value. The
   *     return value will be stored in the form {node: Node, remainder: number}
   *     if this object is provided.
   * @return {Node} The node at the given offset.
   */
  goog.dom.DomHelper.prototype.getNodeAtOffset = goog.dom.getNodeAtOffset;
  
  
  /**
   * Returns true if the object is a {@code NodeList}.  To qualify as a NodeList,
   * the object must have a numeric length property and an item function (which
   * has type 'string' on IE for some reason).
   * @param {Object} val Object to test.
   * @return {boolean} Whether the object is a NodeList.
   */
  goog.dom.DomHelper.prototype.isNodeList = goog.dom.isNodeList;
  
  
  /**
   * Walks up the DOM hierarchy returning the first ancestor that has the passed
   * tag name and/or class name. If the passed element matches the specified
   * criteria, the element itself is returned.
   * @param {Node} element The DOM node to start with.
   * @param {?(goog.dom.TagName|string)=} opt_tag The tag name to match (or
   *     null/undefined to match only based on class name).
   * @param {?string=} opt_class The class name to match (or null/undefined to
   *     match only based on tag name).
   * @param {number=} opt_maxSearchSteps Maximum number of levels to search up the
   *     dom.
   * @return {Element} The first ancestor that matches the passed criteria, or
   *     null if no match is found.
   */
  goog.dom.DomHelper.prototype.getAncestorByTagNameAndClass =
      goog.dom.getAncestorByTagNameAndClass;
  
  
  /**
   * Walks up the DOM hierarchy returning the first ancestor that has the passed
   * class name. If the passed element matches the specified criteria, the
   * element itself is returned.
   * @param {Node} element The DOM node to start with.
   * @param {string} class The class name to match.
   * @param {number=} opt_maxSearchSteps Maximum number of levels to search up the
   *     dom.
   * @return {Element} The first ancestor that matches the passed criteria, or
   *     null if none match.
   */
  goog.dom.DomHelper.prototype.getAncestorByClass = goog.dom.getAncestorByClass;
  
  
  /**
   * Walks up the DOM hierarchy returning the first ancestor that passes the
   * matcher function.
   * @param {Node} element The DOM node to start with.
   * @param {function(Node) : boolean} matcher A function that returns true if the
   *     passed node matches the desired criteria.
   * @param {boolean=} opt_includeNode If true, the node itself is included in
   *     the search (the first call to the matcher will pass startElement as
   *     the node to test).
   * @param {number=} opt_maxSearchSteps Maximum number of levels to search up the
   *     dom.
   * @return {Node} DOM node that matched the matcher, or null if there was
   *     no match.
   */
  goog.dom.DomHelper.prototype.getAncestor = goog.dom.getAncestor;
  
  
  /**
   * Gets '2d' context of a canvas. Shortcut for canvas.getContext('2d') with a
   * type information.
   * @param {!HTMLCanvasElement} canvas
   * @return {!CanvasRenderingContext2D}
   */
  goog.dom.DomHelper.prototype.getCanvasContext2D = goog.dom.getCanvasContext2D;
  