Document motivation

README.md

@@ -2,6 +2,25 @@
 
 Rules for modern CSS.
 
+## Motivation
+
+Many new CSS features landed in browsers over the past few years, including:
+
+- `@layer` and `@scope` for organising and encapsulating styles
+- `@container` queries and units, such as `cqi`, for size-aware components
+- `:has()` for relational selectors
+- `oklch()` for colour palettes with perceptually uniform lightness
+- `light-dark()` for colour-scheme support
+- `clamp()` for fluid type and spacing
+
+These features help us create robust interfaces that adapt to users' devices and preferences. They replace older patterns like fixed breakpoints, which produce interfaces with seams and brittle code.
+
+LLMs are capable of writing modern CSS. They just need a nudge to use it, or they'll default to older patterns. The agent skill does that, flagging them and pointing LLMs (and people) towards the newer alternatives.
+
+Other modern CSS agent skills have appeared since. I created this GitHub org to pool our efforts, as Stylelint did for CSS linting.
+
+## Contributing
+
 The rules are a living document. If you've spotted a rule that's missing:
 
 - [Open an issue](https://github.com/moderncss/skills/issues)