Even if most websites only relies on 50% of the web spec out there, it's likely that many of them rely on a slightly different 50%. That would mean the "common"

Even if most websites only relies on 50% of the web spec out there, it's likely that many of them rely on a slightly different 50%. That would mean the "common" area needed for a good average experience is significantly larger than 50%. I think this is the trap that non-Chromium MS Edge fell into. I don't doubt that they supported a large subset of websites very well, but the average experience was brought down a lot when it randomly broke or massively slowed down on random websites. The MS Edge case also suggests that it's not enough to just support a standard, it must also be reasonably optimized, because websites can be intolerably slow otherwise. I think many websites, especially web apps, assume that the user's browser is quite fast and can chew through a lot of load.