[WebAPI] Add HTMLTemplateElement.shadowrootclonable attribute

[hamishwillee]

Acceptance criteria

• The listed features are documented sufficiently on MDN
• BCD is updated Firefox 125 ships shadowrootclonable browser-compat-data#22856
• Interactive example and data repos are updated if appropriate
• The content has been reviewed as needed

For folks helping with Firefox related documentation

• Set bugs to dev-doc-complete
• Add entry to Firefox release notes for enabled/preview features
• Add/remove entry to Firefox experimental features page for preference/released features

Features to document

HTMLTemplateElement.shadowRootClonable is supported in FF125 and needs to be documented.

Also document HTMLTemplateElement: shadowRootDelegatesFocus and shadowRootMode which are in BCD but not in doc (already in BCD from FF123)

The IDL changes for shadowRootClonable are shown below. I don't know if SetterThrows is relevant.

 [Exposed=Window]
 interface HTMLTemplateElement : HTMLElement {
   [HTMLConstructor] constructor();
 
   readonly attribute DocumentFragment content;
   [CEReactions, Pref="dom.webcomponents.shadowdom.declarative.enabled"]
   attribute DOMString shadowRootMode;
-  [CEReactions, Pref="dom.webcomponents.shadowdom.declarative.enabled"]
+  [CEReactions, SetterThrows, Pref="dom.webcomponents.shadowdom.declarative.enabled"]
   attribute boolean shadowRootDelegatesFocus;
+  [CEReactions, SetterThrows, Pref="dom.webcomponents.shadowdom.declarative.enabled"]
+  attribute boolean shadowRootClonable;
 };

Dunno how this relates to FF122 project item #21948

Related Gecko bugs

https://bugzilla.mozilla.org/show_bug.cgi?id=1880188

Other

• Check content open issues to see if any pertain to the subject matter. If there are any that can be closed because of the work, do so. If there are any that can be fixed relatively quickly because of the knowledge from completing this issue and you have time, feel free to go ahead and fix them.

[hamishwillee]

Status

• BCD
  • Firefox 125 ships shadowrootclonable browser-compat-data#22856
  • HTMLTemplateElement.shadow RootMode/DelegatesFocus/Clonable link to docs browser-compat-data#23011
• Docs FF124 shadowRootClonable and friends #33325
• Release note FF125 Relnote: shadowrootclonable attribute and shadowRootClonable pr… #33431
• dev-docs-complete