CartoCSS for OSM.org's tiles
CartoCSS is used in the rendering process of the "Standard" map tiles of OSM.org since August 2013. The "openstreetmap-carto" CartoCSS stylesheet is maintained at https://github.com/gravitystorm/openstreetmap-carto. Bug reports (e.g. rendering problems of specific object classes) can be raised there (needs a login with a github.com account).
CartoCSS 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.
Though CartoCSS stylesheets can work with any data structure, an 'OSM' carto stylesheet based off of one schema, like Imposm or HighRoad, can be swapped with another stylesheet based on the same schema.
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 CartoCSS
Andy Allan converted the Mapnik XML style of the OpenStreetMap frontpage to Carto: openstreetmap-carto on GitHub
There is also a work in progress adaption of the German Mapnik XML style to Carto: openstreetmap-carto-german on GitHub (not yet running on the tileserver)