modern-html

Baseline-first HTML/CSS guidance for AI coding agents. Routes UI generation toward native HTML elements and modern CSS that has reached Web Platform Baseline Widely-available status for at least 9 months, prompts before Newly-available features, and avoids Limited features without explicit fallback. Covers dialog, popover, anchor positioning, container queries, subgrid, :has(), @scope, @layer, view transitions, scroll-driven animations, light-dark(), oklch(), color-mix(), text-wrap, and accessibility primitives.