Intent to Ship: Declarative shadow DOM: shadowrootslotassignment attribute

[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

Motivation

slotAssignment is fixed at shadow root creation, so components using manual slot assignment could not adopt declarative shadow DOM. The gap was raised in https://github.com/WICG/webcomponents/issues/967 and specified in the HTML Standard via https://github.com/whatwg/html/pull/12267 . Both Gecko and WebKit have implemented the attribute, so shipping in Chromium completes cross-engine support. This feature is opt-in and the default ("named") is unchanged.

Initial public proposal

No information provided

Search tags

shadow, DOM, slot

TAG review

No information provided

TAG review status

Not applicable

Goals for experimentation

None

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: currently experimental, aiming to enable it by default from M151 and then retained as a kill switch (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: https://developer.mozilla.org/en-US/docs/Web/API/HTMLTemplateElement/shadowRootSlotAssignment

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?

The new behavior is opt-in, the default behavior remains unchanged, and pages that do not use the attribute are unaffected.

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

Flag name on about://flags

No information provided

Finch feature name

ShadowRootSlotAssignment

Rollout plan

Will ship enabled for all users

Requires code in //chrome?

False

Tracking bug

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

Estimated milestones

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

Anticipated spec changes

Open questions about a feature may be a source of future web compat or interop issues. Please list open issues (e.g. links to known github issues in the project for the feature specification) whose resolution may introduce web compat/interop risk (e.g., changing to naming or structure of the API in a non-backward-compatible way).

No information provided

Link to entry on the Chrome Platform Status

https://chromestatus.com/feature/5178682139344896?gate=6073478714359808

This intent message was generated by Chrome Platform Status.

[Chris Harrelson]

LGTM1

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[Yoav Weiss (@Shopify)]

I'd appreciate an explainer (even an inline one) that explains what this does and how we expect developers to use it.

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[Daniel Bratell]

LGTM2 (since we are catching up, our documentation requirements are lowered)

/Daniel

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[Alex Russell]

LGTM3

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