As I said, the browser is a new challenge, and I was referring exactly to the problem of presentation flexibility. However, I'm pretty sure that the boxes-and-glue model can handle flexible viewport and font sizes as inputs from the browser, rather than specified by the user in terms of page size.

CSS has boxes, more or less. It does not have glue in any meaningful way, i.e., it does not give a direct way to specify what happens in between the boxes. Pretty much all the things that are difficult to do in CSS have to do with the difficulty of describing what happens in the negative space.