I've installed Opera 140 Developer Edition and JPEG XL works out of the box. That means everyone will have it once 140 becomes stable.

I just tried Opera 140 (the developer edition) and it supports displaying JPEG XL images! It passes the test at https://jpegxl.info/resources/jpeg-xl-test-page.html with flying colors.

Though it could be expected because it's based on the newer Chromium 155 engine, it's quite a big deal that Opera supports the image format that is expected to make most other image formats obsolete.

I would not let it go unnoticed, I would add it to the changelog!