Form with button does not return value as returnValue

[seralo]

ReturnValue is not set to the BUTTON value parameter on form submission.

When we press "Yes" in the following example dialog.close(undefined) is called instead of dialog.close('yes')

<dialog id="dialog">
  <form method="dialog">
    <p>Do you agree with terms of use?</p>
    <textarea class="form-control" disabled>Lorem ipsum dolor sit amet,....</textarea>
    <button type="submit" value="yes">Yes</button>
    <button type="submit" value="no" autofocus>No</button>
  </form>
</dialog>

Proposed fix: in the submit handler accept either 'INPUT' or 'BUTTON' nodeName.