Support is not a boolean.

A proper test page should have HDR images, images testing if 10-bit gradients are posterised to 8-bit or displayed smoothly, etc...

iOS for example can show a JPEG XL image, but can't forward it in iMessage to someone else.

There have been a few other test pages posted in the comments with varying degrees of additional context.

https://jpegxl.info/resources/jpeg-xl-test-page https://caniuse.com/jpegxl