title: HTML class attribute with spaces, it is a W3C valid class?
body: <p>About HTML <em>class</em> attribute, that assigns CSS class (or classes?) to a tag.
  The use of spaces, like in </p>
  
  <pre><code>&lt;tag class="a b"&gt;....&lt;/tag&gt;
  </code></pre>
  
  <p>is valid?</p>
  
  <p>This syntax is used by some web-designers and occurs into exported HTML of Adobe InDesign (tested with versions 5 and 6), and another HTML generation softwares...</p>
  
  <p><strong>It (<code>class="a b"</code>) is a valid W3C syntax?</strong>  What versions of CSS and HTML?
  (starting from which version became valid?)</p>
  
  <hr>
  
  <p>EDIT: a natural <em>subquestion</em> "W3C say how to interpret it?" (it is an "override" or another renderization behaviour?) <a href="https://stackoverflow.com/q/13849655/287948">was posted here</a>. </p>