Flatpak extensions are broken
We lost H.264 support a couple weeks ago when we switched to freedesktop 1.8 SDK. The problem is the org.freedesktop.Platform.ffmpeg extension is only provided for freedesktop 1.6 SDK:
$ flatpak list Ref Options org.gnome.Sdk.Docs/x86_64/master user,runtime com.igalia.Gobby/x86_64/0.4 system,current org.gnome.Epiphany/x86_64/master system,current org.gnome.Fractal/x86_64/stable system,current org.freedesktop.Platform.Icontheme.Adwaita/x86_64/1.0 system,runtime org.freedesktop.Platform.VAAPI.Intel/x86_64/1.6 system,runtime org.freedesktop.Platform.ffmpeg/x86_64/1.6 system,runtime org.gnome.Platform/x86_64/3.28 system,runtime org.gnome.Platform/x86_64/master system,runtime org.gnome.Sdk/x86_64/3.28 system,runtime org.gnome.Sdk/x86_64/master system,runtime
I assume the VAAPI extension is broken as well. The 1.6 extensions have no chance because they were not multiarch; e.g. the GStreamer plugin path has changed to include the host triplet.
I hesitated to report this bug, because fixing the ffmpeg extension means Epiphany will start autoplaying videos again, and that sucks. But I guess we should fix it. :)