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HTML issues tagged as allowfullscreen.
The allowfullscreen attribute is used to allow an iframe to activate fullscreen mode. As a boolean attribute, it should only be declared without any value.
Here is an example of correct usage:
<iframe src="https://example.com" allowfullscreen></iframe>
However, this is now a legacy attribute, and has been redefined as allow="fullscreen", as part of the more general Permissions Policy:
<iframe src="https://example.com" allow="fullscreen"></iframe>
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