not working HTMLTemplateElement Content append after innerHTML result empty string

PR: https://github.com/WebReflection/linkedom/pull/315

const data: DocumentFragment = ...
const template = window.document.createElement('template') as HTMLTemplateElement;
template.content.append(data);

 console.log('innerHTML:', template.innerHTML, 'children:',Array.from(template.content.children));  // innerHTML emptyString

not working..

innerHTML output is empty string

 console.log('innerHTML:', template.innerHTML, 'children:',Array.from(template.content.children));

--log result--
innerHTML: children: [
<ref *1> HTMLStyleElement {
ownerDocument: HTMLDocument {
ownerDocument: null,
localName: '#document',
nodeType: 9,
parentNode: null,
[Symbol(next)]: [DocumentType],
[Symbol(prev)]: null,
[Symbol(private)]: null,
[Symbol(end)]: [Object],
[Symbol(CustomElements)]: [Object],
[Symbol(MutationObserver)]: [Object],
[Symbol(mime)]: [Object],
[Symbol(doctype)]: [DocumentType],
[Symbol(DOMParser)]: [class DOMParser],
[Symbol(globals)]: null,
[Symbol(image)]: null,
[Symbol(upgrade)]: null,
[Symbol(EventTarget)]: [DOMEventTarget]
},
localName: 'style',
nodeType: 1,
parentNode: DocumentFragment {
ownerDocument: [HTMLDocument],
localName: '#document-fragment',
nodeType: 11,
parentNode: null,
[Symbol(next)]: [Circular *1],
[Symbol(prev)]: null,
[Symbol(private)]: [HTMLTemplateElement],
[Symbol(end)]: [Object]
},
[Symbol(next)]: Attr {
ownerDocument: [HTMLDocument],
localName: 'id',
nodeType: 2,
parentNode: null,
ownerElement: [Circular *1],
name: 'id',
[Symbol(next)]: [Attr],
[Symbol(prev)]: [Circular *1],
[Symbol(value)]: 'KQykvzSaQPUDomNDyYpuZNdcxkWCkvHtGvVujfjH-style',
[Symbol(changed)]: false
},
[Symbol(prev)]: DocumentFragment {
ownerDocument: [HTMLDocument],
localName: '#document-fragment',
nodeType: 11,
parentNode: null,
[Symbol(next)]: [Circular *1],
[Symbol(prev)]: null,
[Symbol(private)]: [HTMLTemplateElement],
[Symbol(end)]: [Object]
},
[Symbol(private)]: null,
[Symbol(end)]: {
nodeType: -1,
ownerDocument: [HTMLDocument],
parentNode: null,
[Symbol(next)]: [HTMLElement],
[Symbol(prev)]: [Attr],
[Symbol(start)]: [Circular *1]
},
...
]

I was thus treated with innerHTML.

• template.innerHTM; .. template.content

But I think it needs to be fixed

Start with the TypeScript reproduction in this issue: append a DocumentFragment to template.content, then compare template.innerHTML with template.content.children. Trace the HTMLTemplateElement serialization path and add coverage showing that appended content appears in innerHTML while preserving the reported children result.