Single, freestanding artworks are not in themselves listed as World Heritage. In a broader context, and typically in the interior “decor” of religious buildings in particular, the outstanding universal value (i.e. the world heritage) of a site may consist of a larger or combined works of one or more artists, and not necessarily the built monument itself.
The Mausoleum of Galla Placidia (Ravenna, Italy) and the Cappella Palatina in Palermo (Sicily) are but two examples in Italy of sublime mosaic artworks dating from the 6th and 11th centuries respectively. The building or construction that houses these artworks are of secondary importance, generally speaking. Arguably you can’t have one without the other. The architecture may in fact be intentionally plain on the exterior, like the Cathedral in Albi in France, serving a purpose that is different from that of the interior.