In the CSS Level 1, 2, and 2.1 there is no way to specify if an element X from the namespace Y should be presented differently from the element X from the namespace Z. In the upcoming CSS Level 3, it is possible to differentiate elements by their namespaces. Oxygen XML Author Author supports this CSS Level 3 functionality. For more information see the Namespace Selectors section.
To match the def element its namespace will be declared, bind it to the abs prefix, and then write a CSS rule:
@namespace abs "http://www.oxygenxml.com/sample/documentation/abstracts"; abs|def{ font-family:monospace; font-size:smaller; } abs|def:before{ content:"Definition:"; color:gray; }