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First Seen: 03/11/2024

Last Indexed: 10/21/2024

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Interaction between the Document-Policy *-images-max-bpp directive and a user-agent’s supported image formats is currently unspecified. Next-gen image formats of the present and future include WebP, AVIF, JPEG-XL, and WebP2. With every new format, new compression ratios become possible; however, cross-browser support is inconsistent. That means possible compression ratios also vary by browser. Fewer supported image formats should allow a less aggressive compression ratio in the Document Policy. Unfortunately, browsers’ Accept request headers don’t always report supported image formats, so servers can’t easily compute the best policy for a given browser. Specifying a per-mimetype compression ratio isn’t ideal. Sometimes a PNG can beat AVIF or come close enough to not justify the extra bytes of a element. On a browser with AVIF and PNG support, loaded PNGs should be held to AVIF-level compression expectations. I think the most robust solution would be to offer multiple image-compression policies to a browser; the browser can then pick the policy that matches its supported image formats. For instance: a server could offer a max-bpp-supports-webp , max-bpp-supports-webp-avif , max-bpp-supports-webp-avif-jxl , etc. Unfortunately, this is really wordy and will only grow more complex as browsers adopt new image formats. TLDR: in a web where supported image formats can vary, it’s unclear how *-images-max-bpp and a UA’s supported image formats should interact. This variance warrants a policy more complex than a single global value.