OpenStreetMap has recently switched over to a new rendering server including a revised cartocss stylesheet. More importantly, this new stylesheet is now actively maintained as opposed to the previous standstill.
Now, you may look for a tool to keep track of all the progress made. Here it is! It lets you compare the new osm-carto tiles with a rendering which is almost the same as the old OSM-style.
The reference rendering is provided by the Wikimedia toolserver. So, please note that the reference rendering may not be as up-to-date as osm.org and there may be glitches that are not related to the stylesheet itself.