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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.

I thought the publisher gets a PDF.