The shadowRootCustomElementRegistry property of the HTMLTemplateElement interface reflects the value of the shadowrootcustomelementregistry attribute of the associated <template> element.
Note: This property is not useful for developers, and is only documented for completeness. If a <template> element is used to declaratively create a ShadowRoot, then this object and property do not exist. Otherwise, if an HTMLTemplateElement is created, the value of this property is irrelevant because the object is not a shadow root and cannot subsequently be changed to a shadow root.
A string that reflects the shadowrootcustomelementregistry attribute of the associated <template> element.