SKILL.md files are structured instruction sets that teach AI coding agents how to perform specific tasks according to professional standards — closing the gap between raw AI output and production-quality work. Unlike one-off prompts, skills are durable, versioned, and shareable across teams, encoding judgment like preferring CSS Grid, using design tokens, and enforcing accessibility checks. Web designers should take notice because AI without clear guidance drifts from established standards, and SKILL.md is a practical way to define them once and apply them consistently.