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<!DOCTYPE html>
<html lang=en>
<head>
  <meta charset=UTF-8>
  <meta name=viewport content="width=device-width,initial-scale=1">
  <title>Dynamic async Example</title>
  <style>body{font-family:system-ui,sans-serif;margin:0;padding:1.5rem;background:#f8fafc;color:#334155;line-height:1.7}h1{margin:0 0 1rem;font-size:1.25rem;color:#0f172a}#output{margin-top:.75rem;padding:.75rem 1rem;background:#fff;border:1px solid #e2e8f0;border-radius:8px;font-size:.875rem}.hint{margin-top:1rem;font-size:.8125rem;color:#64748b;max-width:480px}</style>
</head>
<body>
  <h1>Dynamic async</h1>
  <p id=output>Waiting for async script...</p>
  <p class=hint>JavaScript creates a script, sets <code>async = true</code>, then appends it to the document.</p>
  <script>var scriptElement=document.createElement("script");scriptElement.src="data:application/javascript,document.getElementById('output').textContent='Script loaded asynchronously.'",scriptElement.async=!0,document.head.appendChild(scriptElement)</script>
</body>
</html>

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HTML async Attribute — Dynamic JavaScript