A CSSX (Extended Cascading Style Sheets) file including non-CSS (Cascading Style Sheet) extensions is used to define and reference variables and inheritance sets. A CSSX file compiler determines a value of the defined variable, modifies the CSSX file by replacing all references to the defined variable with the value, and generates the CSS file from the modified CSSX file. The inheritance set is defined in the CSSX file and includes a reference to a previously defined CSS rule set. The CSSX file compiler defines a new CSS rule set as a function of the determined attributes included in the previously defined CSS rule set of the defined inheritance set and generates the CSS file including the newly defined CSS rule set.