What is the XCSS file format?
4.4.9. XCSS File Format Cross-site Cascading Style Sheet (XCSS) files are the core of RichFaces component skinnability. XCSS is XML-formatted CSS that extends the skinning process. RichFaces parses the XCSS file containing all look and feel parameters of a particular component and compiles the information into a standard CSS file that can be recognized by a web browser. The XCSS file contains CSS properties and skin parameter mappings. Mapping a CSS selector to a skin parameter can be done with < u:selector > and < u:style> XML tags, which define the mapping structure