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HTML Guide

Bad value “true” for attribute “allowfullscreen” on element “iframe”.

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="" 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="" allow="fullscreen"></iframe>

Learn more:

Related W3C validator issues