We've tried "document modes" before, and it was terrible. And it's even worse for JS and CSS.

> Is it really worth breaking all legacy websites?

In any organization, this question gets asked of their product's technical debt, many times over, and gets many "no" answers, until finally there's too much, and their "technical bankruptcy" is unavoidable. They suffer the (temporary) pains to deal with some or all of that debt, and get back on track.

I'm just wishing we could treat our industry that way. I think we're "there" at that point of decision. I just want to tip us over the edge into admitting it.