[pavlov]

Writing UIs in HTML+CSS is only easy for people who know it already, to be honest. It's not a great UI layer.

I've been using it for almost 20 years now, and I'm still easily confused by the completely unintuitive interactions of CSS parameters like floats and clears, position:relative, box model nuances, etc. (I'm sure that someone is now itching to chime in and tell me I'm not a "real web developer", sort of like the argument on HN the other day about whether "real developers" are allowed to write <br/> ...)

Game development teams don't tend to include CSS wizards. Writing a new API can be a reasonable choice given the expertise available... Not to mention the substantial runtime overhead of loading a complete browser just to display some UI widgets.