It's very possible to have other goals than having a usable and marketable product, Andreas Kling and SerenityOS developpers have explicitly stated other goals on numerous occasions, and other positive side-effects have emerged from the project. These include having something to stay busy long term to stay away from drug use, having fun, learning, prove that things are possible, identifying and reporting mistakes in specifications that were caught thanks to the blank slate implementation approach.
Maybe Ladybird is not usable right away, so what? It's getting impressively close to this point with barely 5 years of development, from scratch (and it goes much deeper than HTML, CSS and JS engine, they also re-implemented the whole networking stack, image/audio/video codecs, font...) and by a small team of mostly volunteers, but more importantly, it's a positive project not only for the developers, but also for their audience and for greater scale web standardization.
I hardly imagine a project such as the one you're describing getting as much traction as Ladybird / SerenityOS (in fact, there are many such of these, but I don't see nearly as much talk and interest for their development), the whole project isn't about the end product, it's explicitly about the process to get there.