If I wanted to write a book, how does this translate to the publisher's standards when it comes time to hand in the design? Do publishers accept css files? How does that whole publishing pipeline work with this css framework?
That would depend very much on your publisher... that could be anywhere from "use this page size and send a PDF" to "manuscripts only and our designer/typesetter/etc. will work on the rest". As someone who has used CSS in the way described here, I think it's beneficial even if you end up having to redo it in InDesign. It may not be a perfect 1:1 but simply coming up with the rules will make setting up styles and master pages in publishing software much more straightforward.
If you're self publishing (like I was), your publisher is probably Amazon, and they take a PDF, which is the output of weasyprint.
I haven't looked at wider distribution, but I believe most of the print on demand publishers accept a PDF file. I think some (all?) also take InDesign, which is Adobe's thing.
Publishers (traditional ones like Random House, HarperCollins) would take a Microsoft Word document manuscript. Each publisher has guidelines for their manuscript.
They will handle the typesetting themselves.
You need to do it all only if you are self-publishing.
