I always have questions to do with web design considering the amount of frameworks out there that you can use, its massive! This site could be for questions ranging from React to bootstrap, HTML to CSS etc.
It could also be used for questions about UI and interfaces. I just can't find the right community that is live right now to ask these sorts of questions, hence the idea of having it's own site.
I saw there was a proposal ages ago, here yet it got closed due to it not having enough activity. But that was six years ago now and I believe the community for it has grown in a large amount and will keep on growing as we all rely more on the web.
I know there's communities like Stack overflow and Web masters, yet I still don't think they fit. Stack Overflow is directed at questions for IDE developers and people with algorithm problems. Web masters is directed for people maintaining systems, operating and managing your own website.
None fits the style of designing websites etc.