Are you getting rid of Web forms now? Also, remember that at the very beginning Berners-Lee's WorldWideWeb browser supported HTML editing, so it's definitely part of some vision of the "good old days".
Yes, if you cut down or eliminate CSS, or just redesign it, you could build a faster renderer.
But these questions point to one of major defects of the "cut-down Web" vision: lots of people have a limited set of Web features that they like, but they tend not to agree on what that set is. You like flexbox. Some people really like grid. Some people really need ruby, or floats, or columns, or editing, or vertical breaking (pagination). Pretty soon you've got something nearly as complicated as real CSS.
Democracy to the rescue:
1. Start a FLOSS mailing list thread to get as much input as possible on the best starting point for this reduced set of functionality. Gopher, resurrecting Xanadu, revisiting Hypercard, and so on. Let the conversation flow.
2. Invite the people who chose Firefox Reader View to private gitlab instance and get to work.
True, general web layout is a hard problem. But there are a lot of knobs to turn. What if we removed CSS layout altogether, and only allowed linear pages, a la reader mode? That's a big tradeoff, but maybe it would work. If that's too much, maybe only allowing flexbox would be enough. As for editing, at that point you're making a web app.