> Web browsers need to composite the video within arbitrary documents which may e.g. have elements that overlap the video (opaque or transparent) or may arbitrarily transform the video (CSS applies to videos just like any other element).

both Kodi and MPV have UI overlays that are composited on top of the video. Also, hardware decoding APIs (vdpau, vaapi...) are mostly unrelated to rendering - see for instance https://github.com/NVIDIA/vdpau-hevc-example/blob/master/mai...