An overview of GNOSIS CMSS, a styling language for both classic and 3D cartography.
Ecere developed the GNOSIS Cascading Map Style Sheets language in conjunction with the OGC Innovation Program's Tetstbed 13 and 14, and the open specifications for it are available in a public Engineering Report: http://docs.opengeospatial.org/per/18-025.html#StylingModel
Key features of GNOSIS CMSS include cascading of styling rules and sheets, any symbolizer value can be a complex expression, and a concise and clear syntax.