feat(core): support custom html elements in head tags

[lebalz]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

see #11570

Test Plan

How to test this? I checked it locally by editing docusaurus.config.ts with all possible combinations...

Tested Configurations

Valid Configs: ✅

{
  headTags: [
    {
        tagName: 'meta',
        attributes: {
          id: 'foo',
        }
    }
  ]
}

{
  headTags: [
    {
        tagName: 'custom-element',
        customElement: true
    }
  ]
}

{
  headTags: [
    {
        tagName: 'custom-element',
        attributes: {
          id: 'foo',
        },
        customElement: true
    }
  ]
}

Invalid Configs ❌

{
  headTags: [
    {
        tagName: 'meta'
    }
  ]
}

{
  headTags: [
    {
        tagName: 'meta',
        customElement: false
    }
  ]
}

Test links

Deploy preview: https://deploy-preview-_____--docusaurus-2.netlify.app/

Related issues/PRs

#11570

[netlify]

✅ [V2]

Name	Link
🔨 Latest commit	4c60a50
🔍 Latest deploy log	https://app.netlify.com/projects/docusaurus-2/deploys/6928248a15752b000890f91c
😎 Deploy Preview	https://deploy-preview-11571--docusaurus-2.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[slorber]

LGTM 👍

I'm adding a little unit test

Also unfortunate that the tag validation logic is spread in 2 different places, but that can be refactored in a separate refactor PR.

[slorber]

I'm going to avoid the union type in docs. Although more accurate, it could make the docs harder to read for the usual case where devs are not using custom elements

Although we can be flexible, I'm not sure what could be the value of adding a custom element in head if it doesn't have any attribute. It would only be a "marker" somehow, but it's not really your use-case. I think it's fine if we keep documenting it as if the attributes were required (the object can be empty anyway)

[slorber]

Not sure why this PR doesn't trigger our CI jobs

Maybe it's because you created the branch from main (it's better to create a dedicated branch in the future).

Anyway, I fixed the test failures and will merge

[lebalz]

Thanks for the additions, suggestions, and advice — I’ll make sure to apply them next time :)

[slorber]

np 👍

The GH actions problem is something else, looks to apply to many Meta repo but nobody told me anything 😅