HTML Code

<!DOCTYPE html>
<html lang=en>
<head>
  <meta charset=UTF-8>
  <meta name=viewport content="width=device-width,initial-scale=1">
  <title>sandbox — empty (locked)</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 .75rem;font-size:1.25rem;color:#0f172a}iframe{width:100%;max-width:320px;height:130px;border:1px solid #cbd5e1;border-radius:6px;background:#fff}.hint{margin-top:.75rem;font-size:.8125rem;color:#64748b;max-width:420px}</style>
</head>
<body>
  <h1>Empty sandbox</h1>
  <p>Scripts inside this iframe are blocked:</p>

  <iframe sandbox title="Locked-down sandbox demo" srcdoc="<!DOCTYPE html><html><body style='font-family:system-ui,sans-serif;padding:0.75rem;margin:0;color:#334155;'><p>Sandboxed content</p><button type='button' onclick='alert(&quot;blocked&quot;)'>Try alert()</button></body></html>"></iframe>

  <p class=hint>Without <code>allow-scripts</code>, the button cannot run JavaScript.</p>
</body>
</html>

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HTML sandbox Attribute — empty sandbox (max lockdown)