The browser landscape today is composed of practically just one browser: Chrome. It is easier than ever to get browsers (read: Chrome) to implement different rendering schemes based on identifiers or descriptors provided by web devs.
Supporting the latest and greatest is a great marketing ploy, browsers (read: Chrome) are incentivized to support them. Browsers that don't will be forced to support them lest they become even further irrelevant.
I think it is a very bad idea for a single browser vendor to deliberately subvert standards process and attempt to steamroll the web with their own proprietary tech!
W3C's hasn't come out with anything HTML for a long time now. What we're calling HTML5 is spec'd by a Google-financed group of individuals (but mostly Chrome devs) collaborating at github.com/whatwg.
The last WHATWG HTML revision W3C has put into recommendation status is WHATWG HTML review draft published January, 2020. Last year's review draft was rejected due to privacy concerns, and also due to long standing issues with HTML5's so-called outlining algorithm ie the spec's written interpretation of heading levels for landmarks in navigation as a left-over from back when Ian Hickson was editor which didn't meet with the reality of what assistive technologies are actually doing. Meanwhile, the WHATWG spec has been edited (by a long-term W3C efitor) but no new consensus for a new W3C HTML recommendation has been achieved [1].
See also, https://wiki.csswg.org/faq#versioning-css-fixing-design-mist...