Working heuristics from print and screen typography, not laws. They flag, they never block. The audience setting tightens or loosens them.
Function guard. Body, body accent and detail carry meaning. A display, script or handwriting face in those roles is flagged. Headlines and headline accents are allowed to perform.
Audience fit. Every curated family carries a voice (neutral, elegant, playful, loud, handmade and so on). An audience lists the voices that read wrong for it; a match becomes a note with the reason.
Contrast, not conflict. Two different families of the same kind with x-height, width and weight within a few percent of each other read as a mistake.
x-height match. A second family inside body text should be within about 6% of the body's x-height, or the size is corrected so it is.
Measure. 45–75 characters per line for reading text; about 66 is comfortable.
Size floors. Body at least 16px on web, phone and documents; 14px in email; 24px on a 1920-wide slide; 40px on a 1080 social post; 28px on a poster; 29px (7pt) on a card. Some audiences raise these.
Leading. Reading text sits at 1.3–1.8; display type at 0.85–1.2.
Colour contrast. WCAG 2 ratios: 4.5:1 for reading text, 3:1 for large text; 7:1 and 4.5:1 for low-vision and signage audiences.
Faux styles. A weight or italic the family does not ship is synthesised by the browser and flagged.
Metrics are measured live from the rendered font: x-height and cap height as a fraction of the em, average lowercase width, and ink density.