Building a browser for the modern standards-based web is effectively impossible, because it costs too much, takes too long, and requires a standing army to keep up with.
We are at an impasse. The standards cannot be deprecated because they are used all over the web, and because they are used all over the web a new browser maker has little choice except forking chromium or firefox. Even microsoft couldn’t afford to keep adding all the standards to their browser engine. Normally the solution for a messy overgrown implementation is a grand reboot. But we can’t do a grand reboot of the web because we cannot get rid of the legacy. The only viable strategy I see to have real browser engine diversity without giving up on compatibility is moving as much of the standards implementation as possible into JS modules, so new browser makers can start with a small engine that loads the publicly hosted standards modules.