The browser might reach 95-97% compatibility eventually. But the last 3-5% gaps are the most stubborn. Many of these 3-5% are not trivial API calls. They might

The browser might reach 95-97% compatibility eventually. But the last 3-5% gaps are the most stubborn. Many of these 3-5% are not trivial API calls. They might be some subtle legacy quirks, timing/rendering things, etc. There is a lot of poorly written historical code that might be illegal by web standards but browsers are forced to support. Established browsers have decades of patches and regression testing.