HTML Code

<!DOCTYPE html>

<html lang=en>

<head>

  <meta charset=UTF-8>

  <meta name=viewport content="width=device-width,initial-scale=1">

  <title>open — details 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 .75rem;font-size:1.25rem;color:#0f172a}details{margin-bottom:.75rem;padding:.75rem 1rem;border:1px solid #e2e8f0;border-radius:8px;background:#fff;max-width:420px}summary{cursor:pointer;font-weight:600;color:#0f172a}details p{margin:.5rem 0 0;font-size:.875rem;color:#475569}.hint{font-size:.875rem;color:#64748b;margin-bottom:1rem}</style>

</head>

<body>

  <h1>details open attribute</h1>

  <p class=hint>The first panel has <code>open</code> — the second is collapsed until you click.</p>



  <details open>

    <summary>Click to toggle</summary>

    <p>This content is visible by default.</p>

  </details>



  <details>

    <summary>Click to toggle</summary>

    <p>This content is initially hidden.</p>

  </details>

</body>

</html>

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HTML open Attribute — details open vs closed