Currently, Sketchfab only supports tangent space normal mapping. Object- or World-space normal maps are not supported, and using them can produce unexpected results on Sketchfab. If you export a valid format with tangent spaces, we keep them. Otherwise, if you include UVs, we generate the tangent spaces.
Tangents are required for some features, such as Anisotropy, to work, which means your model must be UV mapped if you plan to use them.
See also Materials (PBR).