None of this is about mandating or forcing adherence.
Open standards allow interoperability by default. Open standards simplify development. Open standards encourage the creation of new markets. Open standards allow competitiveness that provides the consumer with choice, which is ultimately what a free market economy thrives on.
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.
Open standards allow interoperability by default. Open standards simplify development. Open standards encourage the creation of new markets. Open standards allow competitiveness that provides the consumer with choice, which is ultimately what a free market economy thrives on.