Here are some of the legal issues you will face trying to use clips in an extension.
Relevant sections of the Twitch Developer Agreement
So if you’re monetizing your extension, you must only show clips from channels that have given consent, such as the channel that installed your extension.
Using a browsers <video> player instead of the Twitch Embed for clips would be violating both the requirement to use the embeddable player for all video content, as well as modification of the embeddable experience by stripping out all of the Twitch Player functionality to use the browsers video player.