HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>document vs body count</title>
</head>
<body>
  <header>Header</header>
  <main>Main</main>
  <pre id="out"></pre>
  <script>
    document.getElementById("out").textContent = [
      "document.childElementCount: " + document.childElementCount,
      "body.childElementCount: " + document.body.childElementCount
    ].join("\n");
  </script>
</body>
</html>

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Document vs body childElementCount (Try It)