[janwas]

Yes indeed, for the lossless mode (based on tech by the same author) we're seeing about 45-82% of GIF size, and 60-80% of (A)PNG depending on content.

[JyrkiAlakuijala]

Also other famous compression gurus, including Alex Rhatushnyak and Lode Vandevenne contributed to the lossless side of JPEG XL.