Server Administration > Item Fields > Customizing Rich Content Fields > Removing and Transforming Style Definitions and Elements
  
Removing and Transforming Style Definitions and Elements
If any CSS selector or rich content field references an unsupported selector or element, the invalid style definition or element is silently discarded.
In the Web interface and instances where rich content field data is displayed as a complete HTML page, such as the Items view, the definitions in the CSS file are transformed before the file is applied to the rich content field data.