The best designers I’ve worked with not only understand their own domain but the possibilities and limitations of how it will be executed, because they are also developers.

I've noticed this as well. The best designer I ever worked with was obviously great at design, but also great at css, she rocked! I've seen this more than once, but would estimate less than 10% of designers I've worked with actually took the time to learn css.

I think more designers should learn css, and more engineers should learn at least some basics of design. Then everyone could communicate better and get more done faster.