In the current YouTube iframe (2021), you have to add fullscreen to the allow attribute:
<iframe allow="fullscreen;">
If I understand correctly you have an iframe that contains a second iframe (the youtube one).
Try adding the allowfullscreen attribute to the "parent" iframe.
For full browser support it should look like this:
The "full screen" button was not working in my Chrome browser on Ubuntu.
I was using the media_youtube module for D6. In the iframe it was using a video URL of the pattern //www.youtube.com/v/videoidhere.
I used the theme preprocessing function to make it output > //www.youtube.com/embed/videoidhere
and it immediately started allowing the fullscreen button to work.
In short, try changing the /v/ to /embed/ in the YouTube URL if you're having a problem.
I managed to find a relatively clean straightforward way to do this. To see it working click on my webpage: http://developersfound.com/yde-portfolio.html and hover over the 'Youtube Demos' link.
Below are two snippets to show how this can be done quite easily:
I achieved this with an iFrame. Assuming this DOM is 'yde-home.html' Which is the source of your iFrame.
I've also checked this against the W3c Validator and it validates a HTML5 with no errors.
It is also important to note that: Youtube embed URLs sometimes check to see if the request is coming from a server so it may be necessary to set up your test environment to listen on your external IP. So you may need to set up port forwarding on your router for this solution to work. Once you've set up port forwarding just test from the external IP instead of LocalHost. Remember that some routers need port forwarding from LocalHost/loopback but most use the same IP that you used to log into the router. For example if your router login page is 192.168.0.1, then the port forward would have to use 192.168.0.? where ? could be any unused number (you may need to experiment). From this address you would add the ports that your test environment listen from (normally 80, 81, 8080 or 8088).
I had to add allowFullScreen attribute to the "parent" iframe. The case of the attribute does matter. I don't think Firefox or Edge/IE11 has a browser specific allowFullScreen attribute. So it looks something like this:
This attribute can be set to true if the frame is allowed to be placed into full-screen mode by calling it's Element.requestFullscreen() method. If this isn't set, the element can't be placed into full-screen mode. See Mozilla Docs
Putting allowfullscreen inside iframe tag without setting it to true is already deprecated. The updated answer for this issue which is fullscreen is not available with embedded YouTube videos is to set allowfullscreen to true inside tag:
we can get the code below the video. In the share option, we will have an option embed. If we click on the embed we will get the code snippet for that video.
easiest way to achieve this in 2021 is:
Go to your desired youtube video, click share and pick embed. Copy and paste the code to your html and you are good to go!
Tested this below code on Edge & Chrome. Expands to full browser view, but not full-screen. I had to stick to this workaround as none of the above solutions worked for my case.
This also immediately auto-plays the video muted though.