Experimental snow for stairs, slabs and nodeboxes (to a degree). Due to the very nature of nodes in Minetest, the snow cannot handle all situations. Moresnow is made for roofs (which do look better with a snow cover in winter), slabs, fences etc.
The texture for the winter leaves (the snowy ones) is taken from Moontest, which can be found at https://github.com/Amaz1/moontest/tree/master/mods/moontest