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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>createProcessingInstruction MDN</title>
  <style>body{font-family:system-ui,sans-serif;padding:1.5rem;} pre{border:1px solid #ccc;padding:0.75rem;white-space:pre-wrap;}</style>
</head>
<body>
  <button id="btn" type="button">Create xml-stylesheet PI</button>
  <pre id="out">Click the button</pre>
  <script>
    document.getElementById("btn").addEventListener("click", function () {
      var doc = new DOMParser().parseFromString("<foo />", "application/xml");
      var pi = doc.createProcessingInstruction(
        "xml-stylesheet",
        'href="mycss.css" type="text/css"'
      );
      doc.insertBefore(pi, doc.firstChild);
      document.getElementById("out").textContent =
        new XMLSerializer().serializeToString(doc);
    });
  </script>
</body>
</html>

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Document.createProcessingInstruction — MDN stylesheet (Try It)