It’s not necessary. CSS 2.1 makes up most of the modern web without other modules and it’s not too hard to implement. It’s just that no one writes compositors.

It’s not necessary. CSS 2.1 makes up most of the modern web without other modules and it’s not too hard to implement. It’s just that no one writes compositors. Partial implementation can exist as valid user agents, and most developers don’t even seem to realize you can have a compliant browser that doesn’t render to CSS 2.1 box model specifications.