title: Set feature-policy to iframe after insertion
body: <p>I have this issue when try request fullscreen in Firefox.</p>
  <p><code>Request for fullscreen was denied because of FeaturePolicy directives</code></p>
  <p>I am trying to set the <code>allow</code> attribute in iframe node from <code>allow='autoplay; fullscreen'</code> to <code>allow='autoplay; fullscreen *'</code> in javascript with <code>setAttribute</code> function before the page loaded and it's work well, but when I set it after the page loaded and it's not work. I see DOM changed the value, but it has no effect. I tried to reload the iframe by change the source to the same, but it will go blank after reload.</p>