title: How to validate checkboxes in an AMP form
body: <p>I have a form in AMP that has a set of checkboxes, for example:</p>
  
  <pre><code>&lt;form method="post" target="_blank" name="form" custom-validation-reporting="show-all-on-submit" action-xhr="/some-xhr-action"&gt;
      &lt;span visible-when-invalid="valueMissing" validation-for="favoriteSports"&gt;You must select at least ONE sport.&lt;/span&gt;
      &lt;label for="favoriteSports"&gt;Choose one or more of your favorite sports:&lt;/label&gt;
      &lt;input type="checkbox" required id="favoriteSports" name="favoriteSports" value="football"&gt;Football&lt;/input&gt;
      &lt;input type="checkbox" required id="favoriteSports" name="favoriteSports" value="baseball"&gt;Baseball&lt;/input&gt;
      &lt;input type="checkbox" required id="favoriteSports" name="favoriteSports" value="basketball"&gt;Basketball&lt;/input&gt;
      &lt;input type="checkbox" required id="favoriteSports" name="favoriteSports" value="soccer"&gt;Soccer&lt;/input&gt;
      &lt;input type="submit" value="Submit"&gt;&lt;/input&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>How can I validate that at least one of the favoriteSports checkboxes is checked using AMP?</p>