JPEG XL has now landed in all major browsers, so I figured I'd take a closer look at what it's good/bad at compared to other formats.
For lossy photos on the web, AVIF almost always produces smaller files than JPEG XL. At default effort settings, the best JPEG XL version is typically 15–25% larger than the equivalent AVIF. At maximum effort settings, the gap narrows a little, to typically 6–20%. There are outliers, where JPEG XL is close to, or even beats AVIF, so if you want the absolute best result for a particular image, it's worth trying both. See the F1 image for a typical example, and the weasel for an outlier.
The JPEG XL encoder matters a lot. jixel, a newer JPEG XL encoder, usually produces smaller files than libjxl, except when the image has detailed transparency. The people image is one example.
