<!DOCTYPE html>
<html lang="ja">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width">
<title>Document</title>
</head>
<body>
<div class="js-ibunaisi">A</div>
</body>
<script>
(function () {
const targetElement = document.querySelector('.js-ibunaisi');
const HtmlDoc = `
<h3>xxxxx</h3>
<p>xxxxxxx</p>
`;
// 関数とかにしておくと良いかもしれない。
let dummy = document.createElement('div'); // divでなくても良い
dummy.innerHTML = HtmlDoc;
let dummyChildren = dummy.children;
targetElement.append(dummy);
}());
</script>
</html>
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