title: How to resolve "Allow attribute will take precedence over allowfullscreen" warning?
body: <p><code>Allow attribute will take precedence over 'allowfullscreen'.</code></p>
  
  <p>I'm getting this warning message because I've added an iframe with both <code>allow="fullscreen" and allowfullscreen</code>.</p>
  
  <p><code>allow</code> doesn't seem to work in IE, Edge, or Firefox, according to <a href="https://developer.mozilla.org/en-US/docs/Web/HTML/Element/iframe" rel="noreferrer">https://developer.mozilla.org/en-US/docs/Web/HTML/Element/iframe</a>.</p>
  
  <p>How can I resolve/silence this warning message in a cross-browser compatible way?</p>
  
  <p>Here's a minimal snippet to reproduce the warning message in Chrome:</p>
  
  <pre><code>const iframe = document.createElement('iframe');
  iframe.setAttribute('allowFullScreen', '');
  iframe.setAttribute('allow', 'fullscreen');
  </code></pre>