It is literally impossible to implement the full WHATWG spec, as an independent developer.

You haven't answered the question.

That's because it's a big question. You can make a browser like Dillo, but it won't be able to run web-based banking software. You can make a browser like Konquerer, but it won't be able to use Netflix, or reliably get past Cloudflare walls. So, I'd say… yeah, browser developers are effectively mandated to follow standards – except that (as I said before) it's impossible for an unauthorised developer to implement the full WHATWG spec.

You're being obtuse.

Standards aren't the issue here.