Attributes make sure that all values are given only once while elements allow for (accidental) duplication.
[springmeyer] Any progress or example patches to test for this?
[milovanderlinden] I would have to vote against this one. For interoperability, I would like to see the mapnik config file move more towards the OGC described SLD (Styled Layer Descriptor). And that is all about CssParameters..
[springmeyer] this has been implemented in trunk for the symbolizers which once did use CSSParameters.