HTMLTemplateElement.content

The content property of the HTMLTemplateElement interface returns the <template> element’s template contents as a DocumentFragment. This content’s ownerDocument is a separate Document from the one that contains the <template> element itself — unless the containing document is itself constructed for the purpose of holding template content.

The Node.cloneNode() and Document.importNode() methods both create a copy of a node. The difference is that importNode() clones the node in the context of the calling document, whereas cloneNode() uses the document of the node being cloned. The document context determines the CustomElementRegistry for constructing any custom elements. For this reason, use document.importNode() to clone the content fragment so that custom element descendants are constructed using the definitions in the current document, rather than the separate document that owns the template content. See the Node.cloneNode() page’s examples for more details.