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 <picture> 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.