I'm using epiphany 3.29.91-1-g8f26b7ff3 with WebKitGtk+ 2.21.5 from gnome-nightly's flatpak.
When visiting any video in the domain as.com, like https://as.com/videos/2018/08/17/portada/1534534794_853344.html, it shows the message "El navegador no admite video HTML5/MP4" ("The browser doesn't allow HTML5/MP4 video"). However, I can play videos in Youtube, for example.
I can reproduce 100%
MP4 support has been broken since April or thereabouts... it needs to be fixed in the runtime. See https://gitlab.gnome.org/GNOME/gnome-sdk-images/issues/7.