Sketchfab does quite a lot of processing and compression of textures in order to reduce download times and improve rendering performance.
However, we know that lossy JPEG compression can sometimes cause issues, particularly for normal maps. This is especially problematic for large flat areas with very little deviation in normals. So, we have implemented an option to force using lossless PNG compression for normal maps.
The huge majority of normal maps are not visually affected by compression. Therefore, this lossless option can only be enabled by Sketchfab Staff, and must be enabled for individual models on a case-by-case basis.
A good way to test if your textures are affected is to compare the 3D Settings rendering (always original / lossless textures) to the regular viewer rendering (always compressed textures).
If you think your model is affected by normal map compression artifacts, contact us with the model information and we can reprocess the normal map(s) for you.