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mapbox/mapbox-studio-classic
Mapbox Studio Classic is a desktop app for designing map styles against vector tile sources, built on node-mapnik and CartoCSS. It is the predecessor to Mapbox Studio, and the README now says to start there for any new map, so it mainly serves people maintaining an existing Classic project.
The repo ships prebuilt node-mapnik binaries for Windows, macOS and Linux, so you are not compiling mapnik from source on day one, which is the usual failure point for this stack. Source import covers GeoJSON, shapefile, TopoJSON, SQLite, CSV, PostGIS and GDAL, so most local data can be styled without a conversion step. The CartoCSS editor is a CodeMirror instance with its own language mode and palette helper, and the manual in docs/studio-classic-manual covers the styling model in detail. The app/test directory has export and style tests against expected fixtures, which is more than many desktop tools of this vintage ship.
The last push was in September 2018 and the README points people to Mapbox Studio as the replacement, so this is a maintenance dead end for anything new. The install instructions still say Node v0.10.x, and the native dependency chain (mapnik, node-mapnik, carto) is pinned to that era, so a source build on a current toolchain is likely to fail. Platform support stops at Windows 7, OS X 10.8 and Ubuntu 14.04, all long out of support. CartoCSS does not carry over to the Mapbox GL style spec, so time spent learning it is time spent on a language with little active development. The repo also carries old vendored front-end code (jQuery 2.0.3, an old Backbone, Spectrum) and a committed npm-debug.log in templates, which makes an archived project harder to audit.