I finally got around to writing a demo of what a design tool for continuous typography might look like. Change the width of the viewport on the right, manipulate the graphs on the right, and observe how the paragraph changes.
The tool lets shape a sample paragraph by defining its typographic properties as linear functions of the viewport width. This is a broad simplification of the ideas I talk about above - ideally, you wouldn't be constrained to a single function type and a single input variable. But I think it's enough to prove the point that a continuous approach makes it possible to produce typography that is correct (whatever that might mean in a given application) across the whole range of whichever input paremeters you're considering.
I decided not to have the tool output CSS in its first iteration, though it would technically be possible to replicate the functions it uses with a combination of media queries,
calc() declarations and
The tool uses the freshly-released Source Serif 4 by Frank Grießhammer as a test font, mostly because it has an axis for optical size that's useful here.