I'd appreciate an explainer (even an inline one) that explains what this does and how we expect developers to use it.
On Monday, June 29, 2026 at 11:24:59 PM UTC+2 Chris Harrelson wrote: > LGTM1 > > On Fri, Jun 26, 2026 at 5:34 AM Chromestatus < > [email protected]> wrote: > >> *Contact emails* >> [email protected] >> >> *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 >> <https://issues.chromium.org/issues?q=customfield1222907:%22Blink%3EDOM%3EShadowDOM%22> >> >> *Web Feature ID* >> declarative-shadow-dom >> <https://webstatus.dev/features/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 <http:///features#tags:shadow>, DOM <http:///features#tags:DOM>, >> slot <http:///features#tags: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 >> <https://chromium.googlesource.com/chromium/src/+/main/docs/testing/web_platform_tests.md>?* >> 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 >> <https://chromestatus.com>. >> >> -- >> You received this message because you are subscribed to the Google Groups >> "blink-dev" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to [email protected]. >> To view this discussion visit >> https://groups.google.com/a/chromium.org/d/msgid/blink-dev/6a3e71b6.cff415cc.dde5e.00b1.GAE%40google.com >> >> <https://groups.google.com/a/chromium.org/d/msgid/blink-dev/6a3e71b6.cff415cc.dde5e.00b1.GAE%40google.com?utm_medium=email&utm_source=footer> >> . >> > -- You received this message because you are subscribed to the Google Groups "blink-dev" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion visit https://groups.google.com/a/chromium.org/d/msgid/blink-dev/74d68ddb-b43e-4f45-a484-38e996711d27n%40chromium.org.
