If Original vs Lossless does not look the same, it is because your browser does not do color management correctly. JPEG XL's lossless is mathematically lossless

If Original vs Lossless does not look the same, it is because your browser does not do color management correctly. JPEG XL's lossless is mathematically lossless, so there cannot be any actual difference. What can happen though is that the "Original PNG" doesn't have an ICC profile and gets treated by your browser in a different way than the PNG produced by the jxl decoder. This is a problem of your browser though, not of JPEG XL, and likely the image you see for "lossless JPEG XL" is the correct one.