legacy-image-formats — Feature-Policy / Permissions-Policy Directive

Quick answer: The legacy-image-formats directive is an experimental Feature-Policy control that restricts use of older raster image formats so pages prefer modern, efficient ones like WebP or AVIF. Disallow legacy formats with the response header Feature-Policy: legacy-image-formats 'none'. Violating images may be blocked or flagged. It is part of the unsupported image-policy proposals.
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legacy-image-formats

This policy controls the ability of the document to render images in legacy image formats. These are defined as any format other than JPEG, PNG, GIF, WEBP, or SVG. In documents in which this policy is disallowed, legacy-format images will be rendered with inverted colours.

How to apply this policy

Send the following HTTP header to control the legacy-image-formats policy, and disallow it on all origins:

Feature-Policy: legacy-image-formats 'none'

Affected use case example

HTML

<img
        src='/test-assets/small-image.bmp'
        width="150" height="150"
      />

Output

Waiting for the demo to report a result...

Does it work?

Currently Firefox, and Chromium based browsers, such as Google Chrome, Samsung Internet, and Opera, are the only user-agents to support Feature Policy. The minimum version that correctly recognises the legacy-image-formats policy is:

Mozilla Firefox

Not supported

Google Chrome

68+

Microsoft Edge

Not supported

Apple Safari

Not supported

See also

Discussion