404 layout generates invalid HTML

[chalin]

The current 404 layout causes nested <main> elements to be generated:

  <main role="main" class="td-main">
    <main id="main">
      <div>
       <h1 id="title">Not found</h1>
          <p>Oops! This page doesn't exist. Try going back to our <a href="/">home page</a>.</p>
      </div>
    </main>
 </main>

This is an instance of #907.