Carto stylesheets differ from MapCSS in a number of ways: the most important being that the underlying data representation is not tied to OSM data or any other datasource. They also aim to expose as much Mapnik functionality as possible, be compilable very quickly, and have advanced features like variables, attachments, and filters that can be gradually applied.
Converting to Mapnik XML
You can compile a Carto MML file to Mapnik XML by running (if using tilemill)
cd mapbox-tilemill ./bin/carto ./files/<your project>/<your project>.mml > mapnik.xml
This can easily be done without tilemill as well, you just have to have the carto binary installed.
Mapnik XML converted to Carto
Andy Allan converted the Mapnik XML style of the OpenStreetMap frontpage to Carto: openstreetmap-carto on GitHub