GStreamer has two different plugins for hardware acceleration: GstVAAPI and GstVA. Only the former is currently available in the Gentoo repos (gst-plugins-vaapi). The problem is that this plugin is deprecated and newer versions of WebkitGTK blacklist it by default. (https://github.com/WebKit/WebKit/pull/790) This breaks hardware accelerated video in Webkit-based browsers on Gentoo, which would otherwise work with the new plugin. Please consider enabling GstVA as a build option in gst-plugins-bad (-Dva=enabled) or packaging it as a separate ebuild. Thanks!
Although currently: va: Scrambled image due to not passing DRM modifiers downstream https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/1236 This bug looks like a duplicate of bug 835149.
*** This bug has been marked as a duplicate of bug 835149 ***