Chapter 6 - Typography
Body text is 1rem — the size the USER chose in their browser. Line height is unitless 1.6, so this paragraph and the smaller caption below each compute their own leading from their own font size.
Prose is capped at 65ch. Beyond about 75 characters, eyes lose the line when jumping from the end back to the start — the measure is a readability decision, not a style preference.
A caption at 0.875rem: the floor. Nothing in a real interface should be smaller — readability beats design ambition.