Despite the fix in https://bugs.webkit.org/show_bug.cgi?id=118974 still there are situations where the sound volume jumps to maximum. Depending on the audio device this can at least shock the user. I think pulseaudio flat volumes are a good idea, but WebKit(Gtk) should offer a setting for a maximal volume that is used for elements and also applies to the JS API. This stops propagation of a harmful high system volume. Users can still raise the system volume by hand if needed (pulseaudio then raises the media's volume up by the same rate). Any feedback on this idea? Disabling flat volumes is not a good option because it often requires to open the audio settings and raise an audio stream's volume because two volume levels just get messed up.
CC Calvaris since he understands this issue, but I'm not sure why the resolution would be different now than it was a few years ago. Flat volumes are nowadays globally disabled in all major distributions (Ubuntu, Fedora, Arch) except perhaps Debian (not sure about this one) since the PulseAudio developers agreed that was the best way to resolve this issue.
There's already a set volume API for the page. That won't set a high volume but should scale down the volume that is already there. We don't contemplate changing this because (I already explained it many times) we don't want to break any integration with GNOME/Pulse volumes. It's a known issue yes and as you say, caused by flat volumes.