> The lossless WebP image file sizes was 0,3–0,1 % smaller with the sharp YUV option. This last one is quite surprising as it shouldn’t have any effect on the lossless encoding. There is a tiny positive file size reduction overall with the sharp YUV option but it’s insignificant.
This part was strange and didn't match with my own testing. So I asked libwebp dev (and even checked the code). They confirmed that the Sharp YUV option does not make any difference in lossless (also confirmed by looking at code where this option is only taken into consideration in lossless code path), so either there is some weird bug in libwepb or some other strange case.
See the question and answers here: https://bugs.chromium.org/p/webp/issues/detail?id=567#c6
If you can reproduce this difference size reliably, a dev said they'd welcome a reproducible test case.