Support `sizes=auto` on img and source elements

[johannesodland]

Description

For lazy-loading images, the auto keyword for the sizes attribute tells the browser to use the concrete object size to select a source from srcset, in stead of a predetermined size.

This is very useful for layouts where it is not possible to predetermine the size the image will have ahead of layout.

The specification landed a while ago, but the keyword is currently only supported in Chrome/Edge/Opera.

The auto keyword for the sizes attribute should be included in Interop 2026.

Specification

https://html.spec.whatwg.org/multipage/images.html#sizes-attributes

web-feature

https://web-platform-dx.github.io/web-features-explorer/features/sizes-auto/

Test Links

https://wpt.fyi/results/html/semantics/embedded-content/the-img-element/sizes?label=master&label=experimental&aligned&q=auto

Additional Signals

No response

[github-actions]

This comment was automatically generated based on the information you provided. Please don't edit it.

Below is additional information about the web feature (from the web-features project) which is referenced in your proposal.
If this doesn't accurately correspond to your proposal, please update your initial comment to include web-features: <feature-id>.
To find feature IDs, use the web-features explorer.

Feature <img sizes="auto" loading="lazy">

• ID: sizes-auto
• Name: <img sizes="auto" loading="lazy">
• Description: The sizes="auto" attribute for the <img> HTML element reserves layout space for an image before it loads, avoiding some layout shifts. This attribute only applies to images with the loading="lazy" attribute.
• Baseline status: Limited Availability
• Docs: HTMLImageElement: sizes property
• Chrome use counter: chromestatus.com
• More information: See the web-features explorer

[eeeps]

Here's last year's proposal, with more links to tests, standards positions, and developer sentiment, for reference: #761 (comment)

[yoavweiss]

FWIW, Shopify polyfills this API: https://github.com/Shopify/autosizes

The feature is very useful for us in lowering the download bandwidth out-of-viewport images take. While the polyfill works, it's not ideal - it can force layout and can break over time, e.g. if browser implementations subtly change their implementation details related to image loading.

[wpt-interop]

Thank you for proposing Support sizes=auto on img and source elements for Interop 2026.

We want to let you know it was not selected to be part of Interop this year.

On behalf of the entire Interop team, thank you for submitting it for consideration. We got many more proposals than we could include in this year's project, necessitating some difficult choices. Please note this should not be taken as a comment on the technology as a whole, or our willingness to consider it for Interop in the future. We appreciate the work you put into your proposal, and would welcome your participation in future rounds of Interop.

Posted on behalf of the Interop team.