Carto
Carto is a Mapnik stylesheet pre-processor developed by MapBox and inspired by Cascadenik. The reference parser is written in Javascript (based off of less.js) and optimized for large stylesheets. Carto stylesheets are used by TileMill but also usable using the carto executable included in the package.
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.
Though Carto 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.
