Cross-origin embedded content: uses allowfullscreen
<iframe allowfullscreen src="http://a.featurepolicy.rocks/fullscreen.html">
</iframe>
<iframe allowfullscreen src="http://a.featurepolicy.rocks/fullscreen.html">
</iframe>
<iframe allow="fullscreen" src="http://a.featurepolicy.rocks/fullscreen.html">
</iframe>
<iframe src="http://a.featurepolicy.rocks/fullscreen.html">
</iframe>