I don't know how the web does it. There are so many cool things being added. How do they keep the spec manageable? What if someone wanted to build a compliant web browser from the ground up?
That's not a concern of the people writing the spec, but Ladybird is making an admirable go of it. You can shed a lot of complexity by only focusing on the modern standards, and Ladybird has helped fix several bugs in the standards themselves.