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.'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
The compatibility table in this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.
Desktop | Mobile | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
unsized-media | Chrome
Full support
66
| Edge
Full support
79
| Firefox No support No | IE No support No | Opera
Full support
53
| Safari No support No | WebView Android No support No | Chrome Android
Full support
66
| Firefox Android No support No | Opera Android
Full support
47
| Safari iOS No support No | Samsung Internet Android Full support 9.0 |
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.