Our fill_rgb_row and fill_rgba_row functions implement YUV->RGB conversion that exactly matches dwebp -nofancy. However, libwebp defaults to "fancy" color conversion; web browsers, imagemagick and seemingly anything else that uses libwebp uses the fancy codepath. This results in significant divergences in the final output on some images, for example this one:

magicountry_20210418221542.webp.gz

The difference is big enough to be seen with the naked eye, without boosting the levels: