title: Link cannot appear as a descendant of a link
body: <p>A React.js app gives the warning</p>
  
  <pre><code>Warning: validateDOMNesting(...): &lt;a&gt; cannot appear as a descendant of &lt;a&gt;. 
  See Element &gt; a &gt; ... &gt; a.
  </code></pre>
  
  <p>What does that mean? How can you prevent it? Are nested links illegal in HTML, HTML5 or React.js?</p>