It doesn't work right now and causes broken display. Old versions of Epiphany didn't even try to use acceleration, but the one that just went into amber-phone-staging actually tries and ends up with just garbage on the screen.
Also, remove mobile-user-agent from Epiphany overrides to fix a warning:
No such key “mobile-user-agent” in schema “org.gnome.Epiphany.web” as specified in override file “/usr/share/glib-2.0/schemas/11_librem5.gschema.override”; ignoring override for this key.