Go talk to the browser vendors and have them roll in the CSS L3 PMM[1] specs, which have been in draft for . . . let's see here . . . oh, huh, two decades.

I guarantee the instant - the millisecond - that CSS PMM goes into the browser, that is the second the PDF ecosystem goes poof[2].

Aside from all that, back to the core point, it's quite late in the day to restrict HTML to page-based UX. It's been at least a decade since the "browser" has officially become a sorta-crappy sandbox for what we used to call client-server computing and just-in-time software development. But the thing about software, writing software, is that it's diametrically opposed to language, natural language, like the kind you use in documents. That's why we call one a "formal" language versus what we call "natural" language. The interface of these two, trying to make them the same thing, is an ever-bountiful cornucopia of suffering.

You see this in complex markup constructs, like conditionals and transcludes[3], that cross blades with natural language, i.e., documents. This is a big part of why every WYSIWYG "just like paper" XML editor ever made sucks so horribly. It's not that the developers are all idiots - I know a lot of them, and they're some of the smartest/canniest developers I've ever known - it's that they're taking on a task that is conceptually unworkable. A document markup that supports conditionals, or transclusion, the file stops being a document and becomes a part of a system that makes documents. Oh look another cornucopia what could be inside . .

[1] Paged Media Module

[2] Well, everywhere outside of high end print production, glossy prints, executive whitepapers. But even there, I predict a huge falloff, due to the existential pain of developing custom PDF pipelines.

[3] AsciiDoc (include::[] with tag=/lines= partials; ifdef::[], ifndef::[], ifeval::[], endif::[]); DITA (@conref, @conkeyref, @conrefend, @keyref; blah blah blah blah); S1000D (CIR, applics, whatever your Program Officer says); DocBook (all sorts of stuff); ATA iSpec 2200 (SGML crap); reStructuredText/Sphinx (.. include::, .. literalinclude::; .. only::, .. ifconfig::); and new kid on the block Typst[a] (#include, #import; #if … else expressions).

[3a] Speaking of which, if you want to see what paper markup looks like, I seriously recommend a look at Typst. It's like LaTeX for humans.

https://pagedjs.org is a pretty good polyfill for [1]