HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>getElementById case</title>
  <style>body{font-family:system-ui,sans-serif;padding:1.5rem;} pre{border:1px solid #ccc;padding:0.75rem;}</style>
</head>
<body>
  <div id="main">Hello</div>
  <button id="btn" type="button">Compare main vs Main</button>
  <pre id="out">Click the button</pre>
  <script>
    document.getElementById("btn").addEventListener("click", function () {
      document.getElementById("out").textContent =
        "main: " + (document.getElementById("main") !== null) + "\n" +
        "Main: " + (document.getElementById("Main") !== null);
    });
  </script>
</body>
</html>

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Document.getElementById — case sensitivity (Try It)