Yes, I was, and I respectfully disagree. Browsers today do a lot more, but frequently the support for newer features is so specific to each browser and in some cases so unstable that it is completely useless for real world projects
Correct. That's why you don't use newer features until they're absorbed by the standard.
But the point is that these non-standard-compliant implementation techniques don't break anything in practice, because every browser is tolerant of them and will always remain so because far too much would break otherwise.
What I meant was: if they don't validate. I didn't mean "break" in the sense that they don't work.
You may not care for the practice, but nothing leaves my hands into production until it validates, except when the stakeholder insists on using something that won't.