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17 lines
1.8 KiB
Markdown
17 lines
1.8 KiB
Markdown
---
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title: "Document policy and image compression"
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date: 2022-08-12T17:00:01-07:00
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replyURI: "https://github.com/wicg/document-policy/blob/main/document-policy-explainer.md"
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replyTitle: "Document Policy Explainer"
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replyType: "TechArticle"
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---
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Interaction between the Document-Policy `*-images-max-bpp` directive and a user-agent's supported image formats is currently unspecified.
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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.
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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.
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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.
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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.
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