title: What CSS is used by browsers for styling invalid <input type="email">s?
body: <p>OK, so in HTML5 browsers you can have:</p>
  
  <pre><code>&lt;input class="txt-box" type="email" name="email" rel="required" /&gt;
  </code></pre>
  
  <p>When the email is not in the proper format it puts a red box around it.</p>
  
  <p>My question is: what is the CSS that determines that style?</p>