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Feature-Policy: unsized-media

This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

The HTTP Feature-Policy header unsized-media directive controls whether the current document is allowed to change the size of media elements after the initial layout is complete.

This restriction solves "layout instability" problem caused by providing default dimensions for images whose size is not specified in advance so that image doesn't change size after loading.

Syntax

Feature-Policy: unsized-media <allowlist>;
<allowlist>

An allowlist is a list of origins that takes one or more of the following values, separated by spaces:

  • *: The feature will be allowed in this document, and all nested browsing contexts (iframes) regardless of their origin.
  • 'self': The feature will be allowed in this document, and in all nested browsing contexts (iframes) in the same origin.
  • 'src': (In an iframe allow attribute only) The feature will be allowed in this iframe, as long as the document loaded into it comes from the same origin as the URL in the iframe's src attribute.
    The 'src' origin is used in the iframe allow attribute only, and is the default allowlist value.
  • 'none': The feature is disabled in top-level and nested browsing contexts.
  • <origin(s)>: The feature is allowed for specific origins (for example, https://example.com). Origins should be separated by a space.

The values * (enable for all origins) or 'none' (disable for all origins) may only be used alone, while 'self' and 'src' may be used with one or more origins.

Features are each defined to have a default allowlist, which is one of:

  • *: The feature is allowed by default in top-level browsing contexts and all nested browsing contexts (iframes).
  • 'self': The feature is allowed by default in top-level browsing contexts and in nested browsing contexts (iframes) in the same origin. The feature is not allowed in cross-origin documents in nested browsing contexts.
  • 'none': The feature is disabled in top-level and nested browsing contexts.

Default value

The default value for unsized-media is '*', that is unsized media elements are allowed for all origins by default. The page will re-flow every time an image with unknown dimensions is loaded.

Specifications

Specification Status Comment
Permissions Policy Editor's Draft Initial definition.

Browser compatibilityUpdate compatibility data on GitHub

Desktop
Chrome Edge Firefox Internet Explorer Opera Safari
unsized-media 66
Disabled
66
Disabled
Disabled From version 66: this feature is behind the #enable-experimental-productivity-features preference (needs to be set to Enabled). To change preferences in Chrome, visit chrome://flags.
79
Disabled
79
Disabled
Disabled From version 79: this feature is behind the #enable-experimental-productivity-features preference (needs to be set to Enabled).
No No 53
Disabled
53
Disabled
Disabled From version 53: this feature is behind the #enable-experimental-productivity-features preference (needs to be set to Enabled).
No
Mobile
Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet
unsized-media No 66
Disabled
66
Disabled
Disabled From version 66: this feature is behind the #enable-experimental-productivity-features preference (needs to be set to Enabled). To change preferences in Chrome, visit chrome://flags.
No 47
Disabled
47
Disabled
Disabled From version 47: this feature is behind the #enable-experimental-productivity-features preference (needs to be set to Enabled).
No 9.0

See also

© 2005–2020 Mozilla and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Feature-Policy/unsized-media