Understanding this project 'ChromiumGStreamerBackend' (https://github.com/Samsung/ChromiumGStreamerBackend), this article would be helpful someone who doesn't know structure of HTML5Video.
HTMLMediaElement derives WebMediaPlayerClient and implements pure-virtual function of parent's. Regarding to intefaces of WebMediaPlayerClient, HTMLMediaElement calls functions of WebMediaPlayer in order to connect REAL system-specific implementation. WebMediaPlayer has a lot of pure-virtual functions to abstract most valuable function of MediaPlayer's. WebMediaPlayerGStreamer derives WebMediaPlayer for specifying gstreamer-implementation.