As browsers continue to give developers more low-level control, the less of a problem this is. Take one of your examples, history. This used to be a high-level feature implemented only by the browser and if you built an SPA your users either had a very bad experience or you had get "clever" with various tricks. Now, we have a nice history API.

You say that everyone benefits from the same years of experience and testing but at the same time they're limited to a single vendor. A single idea of how it should work. And a single implementation. Right now if I don't like how validation works/looks that is implemented in JavaScript, I can choose any number of alternatives or implement my own ideas.

The browser should help developers build the best possible solutions, it should not implement those solutions itself.

Browsers themselves have always had weird edge-cases and as a web developer I've always had to work around them (see this article). Sometimes even depending on the browser. The whole idea of an "SPA" was never even imagined by browser makers but developers made that happen.