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 Editor plugin 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;
}