I've asked this question before on SuperUser, but didn't get an answer. So I'll try here again.
I'm using conkeror version 1.0~~pre-1+git150730-1 on debian strech. When I go on youtube, the fullscreen button is not clickable. For some reason youtube prefers HTML5 over flash. The flashplayer is installed and works on other sites.
Maybe conkeror doesnt permit youtube to go fullscreen. I have not found the corresponding option when editing permissions.
When I go on youtube's HTML5 test site , I get support for HTMLVideoElement, H.264, WebM VP8 but not for Media Source Extensions, MSE & H.264 and MSE & WebM VP9. I don't know, if this info helps.
I'm aware of addons that force youtube to use flash, but failed to install them. So I would prefer a solution that uses HTML5.
Your help is greatly appreciated!
This might work for you: in about:config set full-screen-api.enabled to true.