Add shadowrootslotassignment attribute by keithamus · Pull Request #12267 · whatwg/html

keithamus · 2026-03-16T16:20:03Z

Implements the shadowrootslotassignment attribute.

Refs WICG/webcomponents#967

At least two implementers are interested (and none opposed):
- Mozilla: Interested
- Chromium: Assuming supportive based on Add shadowrootslotassignment attribute #12267 (comment)
- WebKit: Supportive: Support shadowrootslotassignment= attribute WebKit/standards-positions#631
Tests are written and can be reviewed and commented upon at:
- Tenative PR: Add tests for shadowrootslotassignment web-platform-tests/wpt#58546
- Untentative PR: Untentative shadowrootslotassignment tests web-platform-tests/wpt#58713
Implementation bugs are filed:
- Chromium: https://issues.chromium.org/issues/493315747
- Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=2023824
- WebKit: https://bugs.webkit.org/show_bug.cgi?id=310090
- Deno (only for timers, structured clone, base64 utils, channel messaging, module resolution, web workers, and web storage): N/A
- Node.js (only for timers, structured clone, base64 utils, channel messaging, and module resolution): N/A
Corresponding HTML AAM & ARIA in HTML issues & PRs: N/A
MDN issue is filed: Add shadowrootslotassignment attribute mdn/content#43753
The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

/indices.html ( diff )
/parsing.html ( diff )
/scripting.html ( diff )

keithamus · 2026-03-16T16:23:18Z

/cc @mfreed7

rniwa · 2026-03-16T16:24:21Z

It seems that we also need a mechanism to specify which nodes are assigned in what order.

keithamus · 2026-03-16T16:34:19Z

It seems that we also need a mechanism to specify which nodes are assigned in what order.

Right. I think that it would be possible to ship the two independently, right? It's possible today to create a manual assignment shadowroot and never assign nodes.

rniwa · 2026-03-16T16:36:37Z

It seems that we also need a mechanism to specify which nodes are assigned in what order.

Right. I think that it would be possible to ship the two independently, right? It's possible today to create a manual assignment shadowroot and never assign nodes.

Maybe? It seems like we want some kind of plan for it before we this feature though.

annevk

You also need to add the IDL attribute for the template element.

mfreed7 · 2026-03-16T16:45:33Z

/cc @mfreed7

This looks great to me! I could finally address this 4 year old Chromium TODO.

Right. I think that it would be possible to ship the two independently, right? It's possible today to create a manual assignment shadowroot and never assign nodes.

+1 - I think this PR provides perfect parity to the imperative attachShadow() call, which just sets the slotAssignment mode but doesn't actually assign nodes. That might be enough, in that it keeps slots from being automatically assigned to slots before the imperative code boots up and assigns them.

keithamus · 2026-03-17T09:14:27Z

Maybe? It seems like we want some kind of plan for it before we this feature though.

AIUI the design described in WICG/webcomponents#967 adds a new attribute to slot elements. To me this stands to reason that these become opt-in, and therefore the design is additive - which is to say if we go with the proposed solution there, I believe they can ship independently.

So I don't think landing this now would close off the design space for us; landing this will not preclude us from the design in WICG/webcomponents#967, and I suspect it doesn't close off any other potential solutions in a way which would still make sense, but I'd be happy to be corrected on this.

keithamus · 2026-04-12T09:40:00Z

I believe I've completed all the necessary steps for this to be merged, so all that's left is an editorial review. Please let me know if there's anything more to do here.

Thanks!

annevk

If you contrast this with https://dom.spec.whatwg.org/#interface-shadowroot you see that slot assignment comes before clonable. I suggest we try to stick to that order.

annevk · 2026-04-12T12:37:50Z

I also noticed that the order is probably not tested as I changed the order in my implementation and still passed all the tests. (There is a test for it somewhere for the other attributes.)

annevk

Looks good modulo final formatting issue.

annevk · 2026-04-14T14:27:15Z

-         <code data-x="attr-template-shadowrootclonable">shadowrootclonable</code>;
-         <code data-x="attr-template-shadowrootserializable">shadowrootserializable</code>;
-         <code data-x="attr-template-shadowrootcustomelementregistry">shadowrootcustomelementregistry</code></td>
+          <code data-x="attr-template-shadowrootslotassignment">shadowrootslotassignment</code>;


The indentation seems wrong here. They all got an additional leading space?

we hardly knew ye

This brings up to date with the specification for declarative shadow roots: whatwg/html#12267. The `shadowrootslotassignment` attribute on `<template>` elements specifies the slot assignment mode used by the declarative shadow root created by the template. Testing: New tests start to pass --------- Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>

Add shadowrootslotassignment attribute

8779a3e

keithamus added agenda+ To be discussed at a triage meeting topic: custom elements Relates to custom elements (as defined in DOM and HTML) labels Mar 16, 2026

annevk reviewed Mar 16, 2026

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keithamus added 2 commits March 17, 2026 09:09

Add template IDL reflection

c78ef20

Reword spec to use attribute state not value

f71532c

This was referenced Mar 17, 2026

Interaction between imperative slotting and declarative shadow DOM WICG/webcomponents#967

Open

Support shadowrootslotassignment= attribute WebKit/standards-positions#631

Closed

sorvell mentioned this pull request Mar 17, 2026

Clarify behavior of calling attachShadow on a declarative shadowRoot #12268

Closed

noamr mentioned this pull request Mar 18, 2026

Upcoming WHATNOT meeting on 2026-03-19 #12275

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noamr removed the agenda+ To be discussed at a triage meeting label Mar 19, 2026

keithamus requested a review from zcorpan April 4, 2026 19:18

keithamus mentioned this pull request Apr 12, 2026

Add shadowrootslotassignment attribute mdn/content#43753

Open

keithamus mentioned this pull request Apr 12, 2026

Add HTMLTemplateElement.shadowrootslotassignment attribute mdn/browser-compat-data#29457

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annevk reviewed Apr 12, 2026

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re-order attribute to before clonable

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annevk reviewed Apr 13, 2026

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keithamus added 2 commits April 13, 2026 16:15

re-order some more

794d13a

fix td

cff9f59

annevk approved these changes Apr 14, 2026

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Be gone spaces!

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we hardly knew ye

annevk merged commit 7fe76d0 into whatwg:main Apr 15, 2026
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keithamus deleted the add-shadowrootslotassignment-attribute branch April 15, 2026 16:35

This was referenced Apr 15, 2026

Add shadowrootslotassignment atom servo/html5ever#736

Merged

script: Support shadowrootslotassignment on template elements servo/servo#44246

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keithamus commented Mar 16, 2026 •

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