When literally every OS already comes with a form of libjxl pre-installed there is no good reason not to add support for it.
> you just truncate the output stream at the right proportion of pixel data
Does the format have explicit support for this? As in, is there any easy way to do, say, an exact Range request after reading the file header?
Worst case scenario you should be able to add some smarts to the server and a query string such that the client can request 5% of the file or whatever simply by fiddling with the url.
The same could be said for .webm, but Apple refused to support it because they wanted to patent-laden alternative to thrive.
Where do people get this stuff?
Apple finally supported WebP in 2020, 10 years after it was released by Google. Firefox added WebP support in 2019, a year before Safari. Was Mozilla also part of conspiracy to undermine WebP? Come on now.
Seems to me neither Apple or Mozilla wanted to be forced to support a format they had no say in developing. Usually there's a process for coming to consensus on standards.
WebP is ubiquitous now, not the case in 2010 when it was released.
It had advantages compared to JPEG, but not enough at the time to justify changing workflows, etc. WebP initially had better compression than JPEG, but as encoders/decoders improved (MozJPEG, Jpegli, turbo-libjpeg, Guetzli) the lead WebP had mostly went away regarding compression and visual fidelity.
Going forward, WebP is becoming less relevant by the day; it doesn't support wide gamut color; it only supports RGB. It's max pixel count is quite limited compared to AVIF and JPEG XL. Newer formats have use cases beyond the web; WebP doesn't.
Loading vs loaded difference needs to be clear.
It's an attack vector for images to have different thumbnail/first bytes than the final image so software still have to decode the whole image.
png/webp (just as jxl) can store thumbnail in their meta, but no operating system, gallery, browser etc uses that because it has no usage that is safe.
I am not sure how easy it is to create a jxl image that starts differently from how it ends. If it is super easy then I expect programs not relying on it for thumbnailing (especially that they already have a way to generate and store thumbnails). Maybe browsers will support progressive loading, but it has no use for local images.