I am not sure how you came to the conclusion that "we all know" , I am still evaluating using HTML based GUI for an HTML5 (canvas/webGL) game.

And for a non-web game, an own (subset/lighweight/compact) parser makes more sense than using imo.

HTML and CSS are better than they were, but they still have years of cruft remaining that will take years more to deprecate and fall out of implementations. (Non web) Games are not required to be rendered by a web browser and have no need to maintain backward compatibility with HTML and CSS, so why not just skip ahead to a better technology?

Seriously, the people coming up with CSS should be shot into the sun. It's such a broken piece of crap that it should be completely replaced.
Unfortunately it's now pretty entrenched and we are going to suffer from it for years to come.

Avoid it, esp. in games. Do you want to show real web pages in your game or simply want to place some buttons?