title: Undefined behaviour in (X)HTML?
body: <h2>Is there such a thing as undefined behaviour in (X)HTML?</h2>
  
  <p>I have wondered this after playing around with the <code>&lt;button&gt;</code> tag, which allows HTML to be rendered as button. Nothing new so far...</p>
  
  <p>But I noticed that one can also use the <code>&lt;a&gt;</code> tag. Complete example:</p>
  
  <pre><code>&lt;button&gt;
      normal text
      &lt;b&gt;bold text&lt;/b&gt;
      &lt;a href="http://www.example.com/"&gt;linked text&lt;/a&gt;
  &lt;/button&gt;
  </code></pre>
  
  <p>This is rendered as following on Firefox:<br />
  <a href="https://i.stack.imgur.com/E7vsg.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/E7vsg.png" alt=""></a>
  
  <br /></p>
  
  <p>And in Google Chrome:<br />
  <a href="https://i.stack.imgur.com/NBru8.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/NBru8.png" alt=""></a>
  
  <br /></p>
  
  <p>Now, On firefox, the link target is <em>NOT</em> clickable, only the button... However, on Chrome, the link is clickable, and will redirect to the <a href="https://www.iana.org/domains/reserved" rel="nofollow noreferrer">IANA RFC2606 Page</a>.</p>
  
  <p>Is this undefined behaviour? Are there more cases in (X)HTML that could be described as undefined behaviour?</p>