title: Fallback for CSS attributes without unit
body: <p>Consider a scenario where CSS attribute is missing the unit (px, em, pt, %):</p>
  
  <pre><code>&lt;body&gt;
      &lt;div 
          style=  "width:170;
                  border:1 dotted PaleGreen;
                  background-color:MistyRose"&gt;
          The quick brown
      &lt;/div&gt;
  &lt;/body&gt;
  </code></pre>
  
  <p><strong>Questions:</strong></p>
  
  <ol>
  <li>Why would it fallback to px? Is pixel always a preferred unit? Is there any rule defined in W3C working draft or recommendation?</li>
  <li>Is there a rule which makes it mandatory for a UA to fallback to a preferred unit?</li>
  <li>Given the above example, which of the following is a correct behavior:
  <ul>
  <li>Internet Explorer: In Quirks mode (IE6,5,4..) width and border-width used to fallback to px. Since IE7 (till present, IE10RP) it ignores the whole rule if unit is missing. Therefore both rules were ignored.</li>
  <li>Firefox 13: In the above example width fallback to px, but border-width was ignored.</li>
  <li>Chrome 19, Opera 12, Safari 5.1.2: Both width and border-width fallback to px.</li>
  </ul></li>
  </ol>
  
  <p>Note: On Microsoft-Connect, they said:</p>
  
  <blockquote>
    <p>The issue you are reporting is by design. IE10 in standards mode ignores a width or height without a unit in compliance with the CSS standards. <em>The unit is not optional.</em></p>
  </blockquote>