HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>PI invalid data</title>
  <style>body{font-family:system-ui,sans-serif;padding:1.5rem;} pre{border:1px solid #ccc;padding:0.75rem;}</style>
</head>
<body>
  <button id="btn" type="button">Try data with ?&gt;</button>
  <pre id="out">Click the button</pre>
  <script>
    document.getElementById("btn").addEventListener("click", function () {
      var doc = new DOMParser().parseFromString("<root/>", "application/xml");
      try {
        doc.createProcessingInstruction("note", "oops ?> still open");
        document.getElementById("out").textContent = "unexpected success";
      } catch (err) {
        document.getElementById("out").textContent = err.name;
      }
    });
  </script>
</body>
</html>

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Document.createProcessingInstruction — ?> in data (Try It)