HTML5 Media Tuner is designed to disable HTML5 Video/Audio completely or just MSE (Media Source Extensions) part of specs selectively per domain using toolbar button. Such disabling is performed only for those functions that are globally enabled in browser settings. Page reload is required to apply the changes.
The easiest way to check how it works is to visit https://www.youtube.com/html5.