# STYLE_REFERENCE_UNDECLARED: CSS uses an undeclared variable or part

> CSS uses an undeclared variable or part

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| Severity | Fault                         | Stage                 |
| -------- | ----------------------------- | --------------------- |
| error    | customer (the source package) | component declaration |

## What it means

A stylesheet of the component reads a CSS variable (`var(--name)`) that the manifest's `cssVars` does not declare and the component's CSS does not define, or names a part (`::part(name)`, `[part=name]`) that `parts` does not declare. The declared variables and parts are the component's whole styling API for the host page, so the build refuses names outside them. Usual causes: a typo, a missing prefix, or a variable used before it was added to `cssVars`.

## How to fix

Use a name the manifest declares, or declare it: a variable in cssVars (with a type and a default), a part in parts.