Not perfectly optimised is fine, but non-functional isn't acceptable. I've never seen a quirk personally, and quirks aren't a good reason to help maintain Google's monopoly on web standards.

You may say less than 5% is fine, but it could be the margins in a low margin industry. 2% could be 40% of the profit.

I haven't seen a team operate where a senior isn't checking it.

Usually the bleeding edge stuff is used by small companies trying to establish themselves because they have nothing to lose and no reputation to protect.

Plus, when you got Browser Stack, you catch a lot of problems like this.