Visiting a webpage contains <audio> or <video> produces:
** (<unknown>:3213): CRITICAL **: 23:09:48.667: gst_gl_context_activate: assertion 'GST_IS_GL_CONTEXT (context)' failed
** (<unknown>:3213): WARNING **: 23:09:48.668: WebKit wasn't able to find the GL video sink dependencies. Hardware-accelerated zero-copy video rendering can't be enabled without this plugin.
It seems that some dependencies are missing. Any suggestions?
Did you install multimedia/gstreamer1-plugins-all? I this is needed for playing audio/video.
These things are handled by the browser engine www/webkit2-gtk3. So it's best to ask there.