I have a theory how this should be solved in web standard, which in turn has other uses besides printing, but well, I don't feel like I can affect web standard in any meaningful sense.
The basic idea is to add (placeholder name) stopUpdating/resumeUpdating to window, which can be polyfilled as no-op. The semantics is that CSSOM view methods are allowed to return the value when update was not stopped, or any later value. That is, current web standard forces you to do things "live". New methods give option to do things in batch.
So long as you don’t read layout information in between, you can already batch your modifications just fine. All it takes is care in how you structure and implement things, and you’re fine. And before you object, your proposed solution would require almost as much care, have more hazards to trip over, and require opting in in a way that few would—and those that would, already know how to be careful. I suspect it would also increase complexity and possibly memory usage in the browser.
If you were designing something from scratch, such approaches would be worthwhile considering, but I think that boat has sailed, and the architecture would fight against you.
Then again, I believe it was generally accepted that web browsers were stuck with UTF-16, until Simon introduced WTF-8 for Servo.
I think the approach of WeasyPrint and Prince, implementing a dedicated layout engine for paged media, is better than making these things work in Browser engines.
In any case, html/css for paged media should be mostly separate from website code. "Printing out" web pages works in many cases, but it's crappy.
The only two reasons why printing web pages out is lousy are because web developers put little to no effort into it, and because the browser manufacturers put little to no effort into it. I would love the likes of WeasyPrint and Prince to be rendered obsolete by one or more mainstream web browsers. If any of them decided that it was a strategic priority, they’d get a lot done very quickly. It’s just that there’s no compelling reason for them to, while there is for the people behind engines with a specific purpose—and so Prince is pretty safe in its position.
The basic idea is to add (placeholder name) stopUpdating/resumeUpdating to window, which can be polyfilled as no-op. The semantics is that CSSOM view methods are allowed to return the value when update was not stopped, or any later value. That is, current web standard forces you to do things "live". New methods give option to do things in batch.