The design.md is basically a designer's style guide, all the sweat, tears and blood of their mastery, in one markdown file that the AI can read.

all the designers, they have a style guide. It's kind of like their guiding principle of their mastery of a designer. Right? You go to school, you you work on 10,000 designs, you have embedded in your blood that design language, that style guide. The design.md is basically all of that sweat, tears, and blood into one markdown file that the AI can read.