There is no need to take sides: there are things that are easier in CSS for sure, and I only highlighted a small number of things which are very nice in TeX box model that are somewhat similar to margins in CSS (non-explicit behaviour to help with smarter laying out of text). TeX has a bunch of problems for electronic documents too (box model it uses means that each individual glyph gets positioned as a separate box, which can result in unsearchable PDFs, for instance).

But TeX typesets to a beforehand known format (I assume you mean target document size) as much as HTML+CSS does (a bitmap canvas of <width> x infinity size): it's not optimized to reflow stuff live, but you can definitely change the page size before every run with a single command, and rendering should (mostly) reflow properly (this would be equivalent to resizing the window, or having differently sized/fine target like a cell phone).

It does not solve the problem of multiple page sizes any better than HTML+CSS, it just provides some nicer and more obvious tools for aligning stuff on the page/screen that could be more easily used to achieve such support.