The only power Safari devs basically have is to decide which OS-supported formats to enable/allow and which ones to disable (e.g. JPEG 2000 is enabled but HEIC isn't).
So getting JPEG XL supported in Safari will most likely first require it to be supported in MacOS and iOS. If you have an Apple device and would like it to get JPEG XL support, then feel free to open a Feedback Assistant ticket (there's an OS-level application to do that) to make a feature request. (I did that 5 weeks ago but haven't heard back yet)