title: HTML5 required attribute one of two fields
body: <p>I have a form with two required input fields:</p>
  
  <pre><code>&lt;form&gt;
      &lt;input type="tel" name="telephone" required&gt;
      &lt;input type="tel" name="mobile" required&gt;
      &lt;input type="submit" value="Submit"&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>Is it possible to get browsers to validate so only one of them is required? i.e if telephone is filled, don't throw an error about mobile being empty and vice versa</p>