GStreamer is broken
GStreamer has been broken since we switched to Freedesktop SDK. All the plugins are missing:
$ flatpak run --command=/bin/bash org.gnome.Epiphany
bash-4.4$ gst-inspect-1.0
staticelements: bin: Generic bin
staticelements: pipeline: Pipeline object
Total count: 1 plugin, 2 features
I tried to fix this in 2fd08a66, but apparently that was not enough.