According to CanIUse, no browser implementation currently supports progressive decoding [1]. This is unfortunate, since progressive decoding theoretically is a major advantage of JPEG XL over AVIF, which doesn't allow it in principle, even though ordinary JPEG allows it. But apparently even a default (non-progressive) JPEG XL allows some limited form of progressive decoding [2]. It's unclear whether browsers support it though.

1: https://caniuse.com/jpegxl

2: https://youtube.com/watch?v=inQxEBn831w