I've been trying to learn how to do design well - struggling to get the onboarding UX for a complex workflow right. It is something very difficult for me to solve well, but a good designer can start iterating in a few days' time. Design is indeed a valuable and deep craft, and my lack of respect for it in the past stemmed from not knowing it and not having worked with masters in the field.
Yes it would be wonderful if designers also understood HTML & CSS - it is quite adjacent to their area of expertise. But is it a deep enough skill if the market doesn't award salaries to people who specialize only in HTML & CSS as much as it does for application developers?
The current designer-developer collaboration workflow is broken, and the dominant narrative blames designers for not learning HTML & CSS. I was also party to it till recently, but it is partly the fault of programmers who don't expend the effort to understand how design is done and what "good" looks like in that craft. It is also the fault of the current crop of tools - vector drawing tools that don't have abstractions nor supports responsive design, and HTML & CSS which is far removed from its visual rendering.
It is a give and take, and it is so much more fun to work when people are eager to learn from each other.