I took a quick look on this web page's source and I found that the structure of the embedded player is iframe A(container) -> document -> embedded iframe B (player) A = <iframe id="xdm_default6411_provider" style="display: block; margin: 0px; padding: 0px; border: 0px none;" scrolling="no" src="https://twitter.com/AAAAAAAAAAAAAAAA" allowfullscreen="" width="100%" height="406" frameborder="0"></iframe> B = <iframe data-src="https://www.youtube.com/AAAAAAAA" scrolling="no" allowtransparency="true" sandbox="allow-popups allow-popups-to-escape-sandbox allow-same-origin allow-scripts" src="https://www.youtube.com/embed/AAAAAAAAAAA" frameborder="0"></iframe> Of course, that is cross-origin and fullscreen is disabled by default in B. And we have to explicitly declare allowfullscreen in B to make B possible to request fullscreen. Another thing I found is Chrome A = <iframe allow="autoplay" allowfullscreen="" src="https://cards-frame.twitter.com/XXXXXXXXX" sandbox="allow-popups allow-popups-to-escape-sandbox allow-same-origin allow-scripts" class="r-1yadl64 r-16y2uox"></iframe> B = <iframe data-src="https://www.youtube.com/embed/XXXXXXXXXX" frameborder="0" scrolling="no" allowtransparency="true" **allow="autoplay; fullscreen"** sandbox="allow-popups allow-popups-to-escape-sandbox allow-same-origin allow-scripts allow-presentation" src="https://www.youtube.com/XXXXXXX"> I could see chrome got allow="autoplay; fullscreen" in B so chrome fullscreen works. Likely, the documents we get from reponses in Firefox and Chrome are diferrents (Firefox without allow fullscreen and chrome with allow fullscreen), and I have no idea about that. I dont think it is a compat issue here, annek what do you think?
Bug 711276 Comment 14 Edit History
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I took a quick look on this web page's source and I found that the structure of the embedded player is iframe A(container) -> document -> embedded iframe B (player) A = <iframe id="xdm_default6411_provider" style="display: block; margin: 0px; padding: 0px; border: 0px none;" scrolling="no" src="https://twitter.com/AAAAAAAAAAAAAAAA" allowfullscreen="" width="100%" height="406" frameborder="0"></iframe> B = <iframe data-src="https://www.youtube.com/AAAAAAAA" scrolling="no" allowtransparency="true" sandbox="allow-popups allow-popups-to-escape-sandbox allow-same-origin allow-scripts" src="https://www.youtube.com/embed/AAAAAAAAAAA" frameborder="0"></iframe> Of course, that is cross-origin and fullscreen is disabled by default in B. And we have to explicitly declare allowfullscreen in B to make B possible to request fullscreen. Another thing I found is Chrome A = <iframe allow="autoplay" allowfullscreen="" src="https://cards-frame.twitter.com/XXXXXXXXX" sandbox="allow-popups allow-popups-to-escape-sandbox allow-same-origin allow-scripts" class="r-1yadl64 r-16y2uox"></iframe> B = <iframe data-src="https://www.youtube.com/embed/XXXXXXXXXX" frameborder="0" scrolling="no" allowtransparency="true" **allow="autoplay; fullscreen"** sandbox="allow-popups allow-popups-to-escape-sandbox allow-same-origin allow-scripts allow-presentation" src="https://www.youtube.com/XXXXXXX"> I could see chrome got allow="autoplay; fullscreen" in B so chrome fullscreen works. Likely, the documents we get in Firefox and Chrome are diferrents (Firefox without allow fullscreen and chrome with allow fullscreen), and I have no idea about that. I dont think it is a compat issue here, annek what do you think?