StyleSheet is the layer underneath, implemented as TeaCSS. It is not a React package and not an Astro package — it is the styling foundation both ecosystems compile against, so a button in an Astro site and a button in a React application resolve to the same tokens, the same scale, and the same colour space.
What it covers
Design tokens, a utility syntax, colour handling in OKLCH, and the visual primitives that would otherwise be re-derived once per project. The parts that are genuinely shared live here exactly once.
What it does not cover
Components. TeaCSS renders nothing and ships no markup. What renders belongs to the ecosystem package for the platform you are building on.