Why is fullscreen is not working if the player is an iframe?

Browsers don’t allow iframes to go in fullscreen per default. To enable this feature, the iframe needs to have the allowfullscreen. For maximum compatibility of browsers, the prefixed webkitallowfullscreen and mozallowfullscreen should be added, too, like this:

<iframe 
  allowfullscreen="allowfullscreen" 
  webkitallowfullscreen="webkitallowfullscreen" 
  mozallowfullscreen="mozallowfullscreen" 
  src="iframe.html" ...></iframe>
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