Precomputing raster tiles became painful once screen densities started to demand @2x and even @3x resolution images. https://github.com/go-spatial/fresco https

Precomputing raster tiles became painful once screen densities started to demand @2x and even @3x resolution images. https://github.com/go-spatial/fresco https://github.com/go-spatial/fresco and https://github.com/maputnik/editor https://github.com/maputnik/editor are TileMill-like editing programs, though specific to one tile format and renderer (MapboxGL)