title: Does < form > break height:100%?
body: <p>Simplest of pages to demonstrate my problem:</p>
  
  <pre><code>&lt;!DOCTYPE html&gt;
  &lt;html&gt;
      &lt;head&gt;
          &lt;style type="text/css"&gt;
              html, body
              {
                  height: 100%;
                  overflow: hidden;
              }
              body
              {
                  margin: 0;
                  padding: 0;
              }
              #container
              {
                  background: red;
                  height: 100%;
                  overflow: hidden;
              }
          &lt;/style&gt;
      &lt;/head&gt;
      &lt;body&gt;
          &lt;div id="container"&gt;&lt;/div&gt;
      &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>div-container correctly fills the browser window with red. Now wrap the div in a form:</p>
  
  <pre><code>&lt;body&gt;
      &lt;form&gt;
          &lt;div id="container"&gt;&lt;/div&gt;
      &lt;/form&gt;
  &lt;/body&gt;
  </code></pre>
  
  <p>and div-container collapses. Why? What is it about a form tag that 'breaks' the 'closest ancester with height'?</p>