Starting off, HTML is not a programming language. CSS is not a programming language. However, it seems that it's being considered a programming language because it is typically done on an editor on a computer.
Here is a search I recently did on CSS through Stack Exchange. The top results are all being asked in Stack Overflow where there are "Q&A for professional and enthusiast programmers". I understand how these results are all on Stack Overflow, and it is probably partially due to how large the community there is. It's possible many other questions are being asked in other Stack Exchange sites but it's not showing up in the top results. However, I think Stack Overflow's community towards programming may not be the correct place for asking questions on things related to web development. Of course, this is just my opinion in this certain case with the question more towards the design put-together. The question I was going to ask was about in which circumstances should I do my designs through software such as Illustrator and After Effects and then bring them into HTML as an image/video, or through CSS explicitly. I personally thought bringing up Adobe Creative Cloud products in Stack Overflow would be considered off-topic.
In other cases, this may depend on the question: There may be ones where it involves JS outside of the design part that needs help. I don't know if other Stack Exchanges are specifically toward web design. Some questions might fit the one for UX or Graphic Design including the HTML/CSS, but my guess is the majority of those communities are there for other questions and answers to help with.
Q: Is this a case then where a Stack Exchange specifically for web design is expected? Or should we continue asking these questions in these existing Stack Exchanges where it still doesn't seem to fit in certain cases?
The reason I ask this is because I view web design as, like the term, a genre of design. I study Media Arts and Sciences which connects studio arts with computer science, and one of the communities I study with are either computer scientists who may do some web design (but mostly using JS for other projects), graphic designers or digital illustration, film/photo editors, or are able to understand both. When I ask those who are more towards programming about web design, they are able to answer the general question but not the details because they are limited with experience specifically towards web design. This may just be an assumption of mine as someone studying MAS. If the majority of Stack Overflow community are comfortable with markup and stylesheet languages just as they are with programming languages, as well as understanding what is considered proper UI design, please let me know. Otherwise, would it be better if I brought this topic to Area 51 as a possible new Stack Exchange community specifically for web design?