Intent to Ship: JPEG XL

In a significant development for web image formats, Google has announced its intention to ship JPEG XL, a new image format that promises substantial improvements over previous formats like AVIF and WebP. JPEG XL, which was first shipped by Chrome in 2021, is set to be supported across major browsers before the end of the year, including Safari, which already has a partial implementation.

The journey to bring JPEG XL to Firefox was not without its challenges. The format, written in over 100,000 lines of multithreaded C++, initially raised concerns about the attack surface it might introduce. To address these concerns, Google challenged the JPEG XL team at Google Research to build a safe, performant, compact, and compatible decoder in Rust. The team rose to the challenge, resulting in the creation of jxl-rs, which now serves as the core of Firefox's JPEG XL support.

Progressive rendering, a feature highly valued by the team, was also mandated as part of an Interop 2026 investigation area. This feature allows images to render progressively as they download, providing users with an initial view of the image even before the full file is received. While Safari's implementation of JPEG XL lacked this feature, Firefox's implementation now includes it, enhancing the user experience.

The choice between JPEG XL and other modern formats like AVIF depends on the specific use-case. At a quality score of 80, AVIF is 227 kB, while JPEG XL requires only 264 kB. At lossless quality, AVIF is 1.76 MB, and JPEG XL is 1.45 MB. In contrast, WebP, a more established format, requires 96 kB at lossless quality.

While AVIF generally produces smaller files at web-quality than JPEG XL, JPEG XL may be preferable for very large images due to its smaller file size at the same quality level. The optimal choice would be to test with a representative set of images for the site, at a quality level that works best for users, and to optimize for high density.