When I read or hear "framework" I get sick.
Especially when it's coupled with a statement like "I don't know HTML or CSS". Why would you try to develop a web app if you don't know the absolute basics?
I specialize in backend development. My first completed project was a backend system I built myself, paired with a frontend generated by AI. So far, it's running smoothly, but I can't fine-tune the frontend precisely since it was AI-generated. That's why I'm eager to learn more about it now.
You're worrying unnecessarily "just in case". HTML and CSS are simple (especially HTML). For HTML basics, any HTML5 book you come across (from O'Reilly, for example) will suffice. For CSS, I would personally recommend Eric Meyer's book describing CSS3. It's very clearly written and includes many examples. It's not about reading it; you simply open it to the chapter that describes what you need. JavaScript can be a bit of a problem. Many people say it's difficult. But in reality, it's just a complete mess (a rubbish project, very sloppily executed) and therefore difficult to work with. Anyone who started with languages like Pascal (or C++ or Java) complains about the primitive nature of JS
*). Unfortunately, there is nothing else on the web browser side. Nevertheless, it can be managed. Current web browsers have JS debuggers. When I was learning JS, there weren't such conveniences

*) It's like living in a comfortable, modern house (with running water, sewage, and electricity) and suddenly having to move to a hut in the woods for an extended period.