Feature-Policy: unsized-media

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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:

  • *: 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.
  • '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
Feature Policy Editor's Draft Initial definition.

Browser compatibility

Update compatibility data on GitHub
DesktopMobile
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung Internet
unsized-media
Experimental
Chrome Full support 66
Disabled
Full support 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.
Edge No support NoFirefox No support NoIE No support NoOpera Full support 53
Disabled
Full support 53
Disabled
Disabled From version 53: this feature is behind the #enable-experimental-productivity-features preference (needs to be set to Enabled).
Safari No support NoWebView Android Full support 66
Disabled
Full support 66
Disabled
Disabled From version 66: this feature is behind the #enable-experimental-productivity-features preference (needs to be set to Enabled).
Chrome Android Full support 66
Disabled
Full support 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.
Firefox Android No support NoOpera Android Full support 47
Disabled
Full support 47
Disabled
Disabled From version 47: this feature is behind the #enable-experimental-productivity-features preference (needs to be set to Enabled).
Safari iOS No support NoSamsung Internet Android No support No

Legend

Full support  
Full support
No support  
No support
Experimental. Expect behavior to change in the future.
Experimental. Expect behavior to change in the future.
User must explicitly enable this feature.
User must explicitly enable this feature.

See also

文档标签和贡献者

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