It ought to be possible to compose HTML documents the way Ted Nelson wanted to compose documents in Xanadu, it's a PITA to do because the classes are all in the same namespace although now we have Shadow DOM to manage this and you can even do it without JS with Declarative Shadow DOM.
It's a source of stress to add a class to the system with a simple, short and readable name and have to worry it is used someplace else. There is BEM but I despite it (not simple, not short, not readable)
I work on a React application that uses more than one plain-CSS framework and also Emotion and some other JS frameworks that generate more CSS properties. These are all leaky abstractions so if you aren't happy with the defaults the way I am with my boostrap-based RSS reader, you have to understand the abstractions of the framework and still understand the underlying CSS. Probably the worst time was when I had to get an application to run inside the Shadow DOM and vendorize some components to patch them accordingly.
One saving grace is that that LLMs are good at both coding CSS and answering questions about how it works.