I don't think the web is slow or bloated, only individual websites are slow or bloated. CSS is absolutely brilliant, and from what I have read, Javascript is the main culprit for badly performing websites.
I don't think a solid argument can be made that because the CSS rendering engine is complicated that it's not performant. Chrome documentation says it's possible to target 60fps with CSS/JS animations.[0]
[0] https://developers.google.com/web/fundamentals/performance/r...