JOSM has a Map Paint Style has a “Mapnik (true)” style
https://josm.openstreetmap.de/wiki/Styles
The standard tile layer at OSM is a Carto Style https://wiki.openstreetmap.org/wiki/Standard_tile_layer but the current version of this style is not available as a Map Paint Style.
I would find it helpful to view how OSM would render my alterations before upload.
Is there a reason why the current Carto style is not available?