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HTML Guide

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Find out what web pages on your sites are affected by HTML issues.

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:

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