[brundolf]

> These aren't things where you can just scan the CSS of the top websites to find out if they're being used.

I guess I was drawing a distinction between a certain subset of functionality that's definitely being used constantly everywhere, like the core box model, vs new features that have gotten layered-on over time and specifically designed not to interfere with or change what came before. For example, the CSS Grid standard has zero effect on any part of page layout unless it is explicitly invoked with "display: grid". These hard barriers were drawn to maintain backwards-compatibility, but they could be leveraged to carve out pieces of functionality to not support, or at least defer support for.

> Building a browser engine from scratch is imho more doable now than it's ever been before

I agree. And I would actually add Rust as a factor for that. Don't forget, Rust was literally purpose-built for building a web browser. With its focus on memory safety and safe concurrency, I'd bet it will act as a very real force-multiplier when it comes to a project like this. Devs will spend that much less time chasing down race conditions and memory errors, while at the same time getting something highly parallel and performant.