It seems like it's very specific to YouTube needs. It supports very few HTML elements and CSS properties [1]. If you think HTML5 subset implies something like HTML4 than you're going to be disappointed. It's very much specialised for div-soup-style apps.
I suspect that list isn't exhaustive. Searching the Cobalt repo shows support for more tags, including <img>. It also supports an on-screen keyboard (likely the platform-specific implementation) as well as game controllers and tilt sensors, so there is support for collecting user input.
It is certainly meant to be a constrained platform for apps. Think of it as a subset of electron, intended for "six foot views" (i.e. television).
You'd obviously need images for implementing a YouTube app (e.g. for thumbnails, icons, avatars), and the HTML subset reference[0] lists jpeg, png and webp as supported formats. Is it possible that images are to be shown using the video tag?
This code yellow thing is so childish, it is almost unbelievable that one of the biggest companies in the world is managed by a bunch of finance guys thinking they are cool tech kids (which is a concept that doesn't exist anymore, grand part thanks to Google becoming IBM).
That's a Google page! The biggest authority on links in the world. Google doesn't need their minions to "protect" its reputation by astroturfing HN. What would be useful is if Googlers could fix this broken link and all their broken systems.
In a few words, Cobalt is a de-bloated Chromium designed to run YouTube TV. It's smaller and faster than launching it in the platform's browser.
If you have a smart TV/set-top box/streaming stick/etc bought in the past few years, the YouTube app is most likely running inside Cobalt. You can develop HTML5 applications that can run in Cobalt, but it supports only a subset of html/css/js stuff you'd expect from a browser.
[1]: https://developers.google.com/youtube/cobalt/docs/developmen...