title: W3C validation says h1 in article is invalid
body: <p>I'm running my website through the W3C validation service. I'm getting an error message for the following HTML:</p>
  
  <pre><code>&lt;section class="about"&gt;
    &lt;article&gt;
      &lt;header&gt;
      &lt;h1&gt;Mission&lt;/h1&gt;
      &lt;/header&gt;
      &lt;div class="content"&gt;
        &lt;p&gt;bla bla bla&lt;/p&gt;
      &lt;/div&gt;
      &lt;aside&gt;
        &lt;img src='/images/logo-hse-250x250.png' /&gt;
      &lt;/aside&gt;
    &lt;/article&gt;
  &lt;/section&gt;
  </code></pre>
  
  <p>The W3C validation error is:</p>
  
  <blockquote>
    <p>Consider using the <code>h1</code> element as a top-level heading only (all <code>h1</code> elements are treated as top-level headings by many screen readers and other tools).</p>
  
  <pre><code>&lt;h1&gt;Mission&lt;/h1&gt;
  </code></pre>
  </blockquote>
  
  <p>I thought with HTML5, it was allowed to use more than one <code>h1</code> tag on a page. And that <code>h1</code> tags could be used inside <code>article</code> elements.</p>
  
  <p>Does anyone have a clue why W3C isn't validating this HTML?</p>