Web-Facing Change PSA: shadowrootslotassignment attribute for declarative shadow DOM

[Chromestatus]

Contact emails

felip...@igalia.com

Specification

https://html.spec.whatwg.org/#attr-template-shadowrootslotassignment

Summary

Adds the shadowrootslotassignment attribute to the <template> element, allowing declarative shadow roots to use manual slot assignment. Until now this option was only available imperatively, via attachShadow({slotAssignment: "manual"}), so components that rely on manual assignment could not create their shadow roots declaratively. The attribute accepts "named" (the default, preserving current behavior) and "manual", and is reflected by the shadowRootSlotAssignment property on HTMLTemplateElement.

Blink component

Blink>DOM>ShadowDOM

Web Feature ID

declarative-shadow-dom

Risks

Interoperability and Compatibility

Low risk. The new behavior is opt-in: it only applies when a page explicitly sets shadowrootslotassignment="manual", and the default ("named") matches current behavior, so existing content is unaffected. The attribute is specified in the HTML Standard (merged via https://github.com/whatwg/html/pull/12267, originally raised in https://github.com/WICG/webcomponents/issues/967) and both Gecko and WebKit have already implemented it, so shipping in Chromium completes cross-engine support. The change is guarded by the ShadowRootSlotAssignment feature flag (enabled by default from M151), retained as a kill switch and expected to be removed around M153.

Gecko: Shipped/Shipping (https://bugzilla.mozilla.org/show_bug.cgi?id=2023824)

WebKit: Shipped/Shipping (https://bugs.webkit.org/show_bug.cgi?id=310090) https://github.com/WebKit/standards-positions/issues/631

Web developers: Positive (https://github.com/WICG/webcomponents/issues/967)

Other signals:

WebView application risks

Does this intent deprecate or change behavior of existing APIs, such that it has potentially high risk for Android WebView-based applications?

No information provided

Debuggability

No new DevTools work required. The attribute is visible in the Elements panel like the other shadowroot* attributes, and the resulting shadow root's mode is already exposed via ShadowRoot.slotAssignment.

Will this feature be supported on all six Blink platforms (Windows, Mac, Linux, ChromeOS, Android, and Android WebView)?

Yes

Is this feature fully tested by web-platform-tests?

Yes
https://wpt.fyi/results/shadow-dom/declarative/declarative-shadow-dom-repeats-slot-assignment.html https://wpt.fyi/results/shadow-dom/declarative/declarative-shadow-dom-slot-assignment.html https://wpt.fyi/results/shadow-dom/declarative/declarative-shadow-dom-slot-assignment-serialization.html

Tracking bug

https://issues.chromium.org/issues/493315747

Estimated milestones

Shipping on desktop	151
Shipping on Android	151
Shipping on WebView	151

Link to entry on the Chrome Platform Status

https://chromestatus.com/feature/5203283359694848

This intent message was generated by Chrome Platform Status.