Don't rely upon it to exist.
Serve up well marked content, CSS that might format it nicely, and JavaScript that allows some types of per-validation to happen client side but only as a means of making a page nicer. The server side should STILL perform all data validation, normalization, and verification (assume someone will craft their own form submission).
Twitch could be done with a native app... the phone versions SHOULD be done with such if they aren't.
As another poster commented, the video aspect can be fulfilled with a number of different methods, and I don't see any technical reason a standard couldn't be followed to allow for arbitrary clients to handle that interaction as well.
As for video streaming, that's not hard at all, especially now that there's a <video> tag.
It's not one of those sites where the content can be easily found in the source of the page either. The latter appears to be, at a cursory inspection, consist of tracking scripts, furthering the irony of this demonstration.
At least a site that calls itself "the TLS" is served via HTTPS...