text: // NOTE: this selector MUST *never* be used directly,
  // always go through query/focusable or is/focusable.js
  // there are too many edge cases that they could be covered in
  // a simple CSS selector…
  
  import selectInShadows from '../util/select-in-shadows';
  
  import _supports from '../supports/supports';
  let supports;
  
  let selector;
  
  export default function() {
    if (!supports) {
      supports = _supports();
    }
  
    if (typeof selector === 'string') {
      return selector;
    }
  
    // https://www.w3.org/TR/html5/editing.html#sequential-focus-navigation-and-the-tabindex-attribute
    selector = ''
      // IE11 supports.can focus <table> and <td>
      + (supports.focusTable ? 'table, td,' : '')
      // IE11 supports.can focus <fieldset>
      + (supports.focusFieldset ? 'fieldset,' : '')
      // Namespace problems of [xlink:href] explained in https://stackoverflow.com/a/23047888/515124
      // svg a[*|href] does not match in IE9, but since we're filtering
      // through is/focusable we can include all <a> from SVG
      + 'svg a,'
      // may behave as 'svg, svg *,' in chrome as *every* svg element with a focus event listener is focusable
      // navigational elements
      + 'a[href],'
      // validity determined by is/valid-area.js
      + 'area[href],'
      // validity determined by is/disabled.js
      + 'input, select, textarea, button,'
      // browsing context containers
      + 'iframe, object, embed,'
      // interactive content
      + 'keygen,'
      + (supports.focusAudioWithoutControls ? 'audio,' : 'audio[controls],')
      + (supports.focusVideoWithoutControls ? 'video,' : 'video[controls],')
      + (supports.focusSummary ? 'summary,' : '')
      // validity determined by is/valid-tabindex.js
      + '[tabindex],'
      // editing hosts
      + '[contenteditable]';
  
    // where ShadowDOM is supported, we also want the shadowed focusable elements (via ">>>" or "/deep/")
    selector = selectInShadows(selector);
  
    return selector;
  }
  