Bug 299299 – customElementRegistry.initialize does not initialize elements as expected

• Status: NEW
• Component: DOM

[Steve Orvell]

When `customElementRegistry.initialize` is called on elements in a declarative shadowRoot with no registry, it does not initialize the elements as expected.

See reproduction: https://codepen.io/Steve-Orvell/pen/RNrNjXr?editors=1000

The expectation is based on this part of the spec: https://html.spec.whatwg.org/multipage/custom-elements.html#dom-customelementregistry-initialize

"For each inclusive descendant inclusiveDescendant of root: if inclusiveDescendant is an Element node whose custom element registry is null:
Set inclusiveDescendant's custom element registry to this."

[Radar WebKit Bug Importer]

<rdar://problem/161577652>