# API — @responsivejs/runtime The authoring half: reactive `value = f(width)`, CSS-first. Depends only on `@responsivejs/core`. Subpaths: `/signals`, `/curves`, `/layout`, `/typography`, `/geometry` (everything is also re-exported from the root). **The entry point is `r$`** — one callable namespace carrying the everyday authoring surface (`r$.fluid`, `r$.tokens`, `r$.geometry`, `r$.whenWraps`, `r$.breakpoints`, `r$.sync`, …), so the editor's autocomplete is the API browser. `responsive` is an alias of the same object (the historical name). Precisely: every `r$` member exists, plus root named exports for the same functions and for the lower layers. The namespace is a **superset of the authoring surface**: transforms, curve sugar, `layout`/`typography` helpers and the measurement signals are all on `r$.` too. Only module-level internals (`emitCSS`, `injectStyle`, `registerProvenance`) stay named-export-only. New to the runtime? Start from the [guide](../guides/runtime), not this reference. ## `r$()` — apply styles ```typescript r$(target, map): ResponsiveHandle r$`selector { prop: ${r$.fluid(14, 24)}px }` // tagged-template form ``` - `target`: selector string, `Element`, `Element[]`, or `NodeList`. - `map`: `Record` where a value is a `ResponsiveValue`, a `(width) => value` function, or a plain string/number. - With a selector target and `useMediaQueries` on (default), the map is **split**: statically expressible values become one injected `