tl;dr: Benchmarking JPEG XL (via libjxl and the newer jixel encoder) against AVIF across photos, screenshots, illustrations, transparency, HDR, and progressive rendering shows AVIF wins in most web scenarios—producing files 15-25% smaller for lossy photos, 30-90% smaller for screenshots/illustrations, and decoding 2-3x faster. JPEG XL's main strengths are lossless compression, transcoding existing JPEGs (9-20% smaller with no quality loss), and occasional outlier images. The author recommends defaulting to AVIF 4:4:4 at high effort, falling back to JPEG XL via jixel only when AVIF produces unusually large files.