The role of the CSS style sheet in XXE is to make the XML document easy to read (get rid of the tree view, no visible tags, etc) and to make its structure (chapter, section, list, list item, etc) easy to understand.
This is very different from the role of CSS style sheets in Web browsers, for which the CSS standard has been designed.
In practice, this means:
You really need to design a CSS style sheet specifically for XML authoring. For that, no need to be WYSIWYG, that is,
you should not try to emulate what will be displayed in the browser after the conversion of the XML document to HTML;
you should not try to emulate what will be displayed in Acrobat™ Reader after conversion of the XML document to PDF.
Note that XXE supports enough CSS to make your XML documents look WYSIRN (What You See Is Really Neat).
Unless you are styling XML data (or a mix of XML document/XML data) rather than XML documents, you should restrict yourself from using XMLmind proprietary extensions. You can style 99% of any type of XML document using the subset of CSS2 supported by XXE. (The remaining 1% is solved by the image() or the image-viewport() content objects.)