The reason is because tmpl.content is an element, and you can’t append an element to another source when it is already appended to something. Try doing this instead:

let elem = document.createElement("div");
elem.append(tmpl.content);
document.body.append(elem);
let elem2 = document.createElement("div");
elem2.append(tmpl.content);
document.body.append(elem2);

It won’t work, because tmpl.content has already been appended to something else.

This is an example of using <template> element.

In the below code, I just wrote a div element inside a template element

  <template id="tmpl">
     <div class="message">Hello, world!</div>
  </template>

And then I just used that template in the script below, a normal use case of template

<script>
  let elem = document.createElement('div');
  elem.append(tmpl.content.cloneNode(true));
  document.body.append(elem);
</script>

We can see that template.content (basically a document fragment) was cloned before using it.
And explanation on javascript.info says that:
Template content was cloned to reuse it multiple times

My question is why we can’t reuse it without cloning?. Can you give a deep insight into this?