@blub It depends what / how you will visualize. You can already use OSM base layers or vector tiles layers draped on a DTM ( see e.g. Mapbox example ).
OSM does not have 3D geometry. Elaborate 3D buildings would have to be converted from OSM semantics to geometry somehow, we could implement that in #PostGIS or #py3dtiles for example .
Next feature coming to #Giro3D is in-browser 2D->3D extrusion for WFS, could be usable for 2D OSM data too. @autra