Contrast

Difference put to work: light against dark, heavy against fine, still against loud. Without it everything competes and nothing is heard.

Contrast is a relationship, not a property. Nothing is heavy until something beside it is fine; nothing is dark until the surface around it is light. Designing with contrast means tuning pairs rather than elements, and deciding where the eye should snag first.

The common failure is medium everything: medium weights, medium sizes, medium greys, a page of polite similarity. We push differences further than feels safe in the file, because screens and print both flatten them. If two elements differ, they should differ visibly; if they cannot differ visibly, they should match exactly.