Just pretend that magically everybody knew how to code. Wouldn't designs just be done in the browser? No longer do you have to maintain these design artifacts post handover. There would be no unrealistic designs that do not consider the constraints of a browser window.
I know this is unrealistic but I often think about this.
Also, they spend their time analyzing the problems and have many other things to hold in their head beside coding. It would be very challenging nowadays to both be a great UX designer and be current with the latest CSS evolutions.
That said, I wish more designers would try coding at some point and learn how a browser and website work to get a deeper understanding of the material they design for.
Also developers should understand the designers methods like journey maps, basic typography and layouting principles.
Designers are supposed to be fluent in the medium they are designing for. Without understanding what's possible, what's easy, what's hard, what squeezes and what scales, or what's going to have a negative impact on site performance, how can one be a great designer?
But then I, as a developer, have to argue with the designer why a table containing a thousand rows or more, with no pagination, may not be a good idea :-(