I find some questions talking about disabling xscreensaver with flash but no one about HTML5. Then, how to disable xscreensaver when firefox is playing an HTML5 vidieo and, if possible, only for some domain?
This question is now 3 years old so the situation might have changed a bit but the issue still exists. I'm facing it on my setup (Debian + Mate). Watching a HTML5 video, even in full screen mode, does not inhibit the screensaver.
This was supposed to be fixed in Firefox 33 (oct. 14) (see bug 811261), but it seems to be more complicated than expected because there are different ways (APIs) to manage inhibition requests.
A bug about this is open (1168090). A patch was proposed a while ago but the development seems stalled.
Disabling the screensaver manually is a bit cumbersome. A simple workaround is to launch vlc when watching a long HTML5 video (even in non-fullscreen mode and even paused, vlc inhibits the screensaver).
There is a sh script called lightsOn that aims at detecting applications that should inhibit the screensaver and reset the screensaver timer on a regular basis when those applications are detected. The original project seems unmaintained, but there are a few forks still alive. From a quick try, partizand/lightsOn seems to do the trick. Basically, download the script and launch it on session startup (as unprivileged user).