alt Hacker News> though their implementation doesn't support progressive images yet
It doesn't support progressive images or it doesn't support progressive decoding of images?
The former would be a massive compatibility issue, the latter would only be a quality of implementation issue.
In terms of AVIF, only Chrome supports progressive decoding. In terms of JPEG XL, Firefox and Chrome support progressive decoding, Safari does not.
In all cases, the browsers are able to render the final image, they just don't show the intermediate stages.