That’s about a 50% reduction in size, lossless, zero reduction in quality or change to pixels from lossless PNG to lossless JXL.
So the browser tools i built basically serves you the tools to easily convert stuff without needing to know anything about libjxl.
The JPEG reconstruction is the most interesting thing, you’re taking a lossy JPEG, converting it to lossless JPEG XL, and you’re seeing a 20% reduction in size while preserving that compression data.
It’s a compressed WebP so what you’re seeing isn’t truly the actual images. Unfortunately I had to upload a compressed WebP of the example, solely to display the size difference.
If you want to see clear comparisons go to https://jpegxl.info/