The CSS module started as a community module, that is, an unofficial set of features that are not fully supported by the official releases and may or may not work. The CSS extension is an example of a successful community module that grew fast and has recently graduated as a formal extension.
Tip
Extensions are an optional GeoServer functionality that are held to the same quality control and documentation standards as WMS and WFS.
The GeoServer download page offers several extensions. Some focus on data access, providing support for the new format for feature and raster data; others add new functionality. The CSS extension is listed under the Miscellaneous category in the GeoServer download page (http://geoserver.org/release/stable/). Please ensure that you download a version of the extension that exactly matches the GeoServer version you are using.