I don't really see anything wrong with a browser that doesn't fully support every website and every weird example of HTML and CSS that a developer decided to hack together. If a browser supports the standards well and does a pretty good job of rendering modern websites then I'd happily trade failing to render some ancient HTML for a web browser that was truly open and free and doesn't send swathes of data about me back to some corporation.
Web browsers are compatible with old HTML because we choose to make them that way. That is a choice. If the cost of having a free and open browser is a requirement to drop some backwards compatibility with old HTML then I think that's actually quite a simple choice. After all, if people want to view an old website then they can still use Chromium or something. It's not like people have to pick a single browser and stick with it.