This is not about the browsers, but about the content the server delivers. When you use one of those limited browsers today, you will encounter a lot of broken pages.

But if there would be place in the internet where every page would stick to the same limited feature set, the new browsers could focus on those features and there users would new a place where they would not encounter broken pages. In addition, users of the traditional browsers (probably the majority of users), would still be able to visit that place too.