I said it in the title, I’ll say it again, SLDs are not just for ugly maps.
I’ve heard from several professionals in this geospatial sector that CartoCSS is so cool– “look at the great cartography you can get out of it”. And, truth be told, creating a standard for cartography that aligns so well with existing web development standards is a brilliant way to get designers in on the map development process (as demonstrated aptly by e.g. Stamen), something sorely needed since Geography decided that cartographers weren’t needed anymore, some time in the late 90s/ early double aughts. That said, and at the risk of sounding parochial, beautiful maps can be produced using SLDs in GeoServer too. The below map is a snapshot from our GeoServer instance, and evolved from a set of cartographic conventions we’ve been developing in our shop, adapted in 4 hours by a darn smart part time employee (Tom Kraft) who had never built an SLD before.