OpenGeo is excited to share the GeoServer CSS module, an alternative to SLD for thematic map styling in the OpenGeo Suite. Compared to SLD, GeoServer CSS brings many advantages for web cartographers.

GeoServer CSS is customized for use in GeoServer and provides access to spatial utilities such as powerful filter expressions that are useful not only for choosing which features to render but also for transforming geometries and other attributes on-the-fly during the rendering process. Thanks to its custom syntax (which builds on OGC CQL the same way that SLD builds on OGC Filter Encoding XML) it is much more compact, allowing authors to see more of the style without scrolling and to make edits without worrying about details such as closing nested XML tags in the proper order. GeoServer CSS is also simpler because it eschews some non-styling aspects of SLD such as embedded geometries.

As an example, let’s say we are styling polygons representing the habitats of several bird species. We would like to distinguish species by color (orioles have an orange area, bluebirds have a blue one, etc.), but all polygons should have a consistent opacity to help us identify places where habitats overlap. A style in CSS notation might look like this:

[species="bluebird"] {
    fill: blue;
    fill-opacity: 70%;
}

[species="oriole"] {
    fill: orange;
    fill-opacity: 70%;
}

[species="robin"] {
    fill: brown;
    fill-opacity: 70%;
}

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