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.
That sounds like a reasonable estimate for the gap between any pair of Chrome, Firefox, and Safari, just from varied depth of implementation of more recent standards.
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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.
That sounds like a reasonable estimate for the gap between any pair of Chrome, Firefox, and Safari, just from varied depth of implementation of more recent standards.