Re-enable custom element test and fix "undefined" child (#3095)

* Re-enable custom element test and fix "undefined" child

* Remove outdated comment

* Adds a changeset

2 files changed, 20 insertions, 0 deletions

diff --git a/packages/webapi/src/lib/CustomElementRegistry.ts b/packages/webapi/src/lib/CustomElementRegistry.ts
index 3f7020087..3429e1e73 100644
--- a/packages/webapi/src/lib/CustomElementRegistry.ts
+++ b/packages/webapi/src/lib/CustomElementRegistry.ts
@@ -26,6 +26,11 @@ export class CustomElementRegistry {
 		if (!/-/.test(name))
 			throw new SyntaxError('Custom element name must contain a hyphen')
 
+		_.INTERNALS.set(constructor, {
+			attributes: {},
+			localName: name,
+		} as any)
+
 		internals.constructorByName.set(name, constructor)
 		internals.nameByConstructor.set(constructor, name)
 
diff --git a/packages/webapi/src/lib/Element.ts b/packages/webapi/src/lib/Element.ts
index 5827a0be0..1553f2863 100644
--- a/packages/webapi/src/lib/Element.ts
+++ b/packages/webapi/src/lib/Element.ts
@@ -1,6 +1,21 @@
 import * as _ from './utils'
 
 export class Element extends Node {
+	constructor() {
+		super();
+		
+		if(_.INTERNALS.has(new.target)) {
+			const internals = _.internalsOf(new.target, 'Element', 'localName')
+			_.INTERNALS.set(this, {
+				attributes: {},
+				localName: internals.localName,
+				ownerDocument: this.ownerDocument,
+				shadowInit: null as unknown as ShadowRootInit,
+				shadowRoot: null as unknown as ShadowRoot,
+			} as ElementInternals)
+		}
+	}
+
 	hasAttribute(name: string): boolean {
 		void name