Typography is the craft of endowing human language with a durable visual form, and the typographer is therefore a practitioner of a language that has to survive the reader.
A variable font stores a continuous axis of design in a single file: the outlines for every weight are interpolated at render time, so a single family can answer 100 and 900 with the same bone structure rather than two unrelated drawings.
This page has no variable font. Its weight slider fakes the axis by pairing font-weight with a synthetic stroke width — a reminder that the illusion is convincing long before the technology is available.
Optical size, width, grade, slant: each axis is a promise about context. The axes interact — a wider face needs a looser tracking, a heavier grade needs more leading — and the craft lies in the negotiation between them.
The three controls at the top of this page are a small model of that negotiation: weight, tracking and leading, each of which changes the meaning of the others.