HTML Code

<!DOCTYPE html>

<html lang=en>

<head>

  <meta charset=UTF-8>

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

  <title>onoffline — banner</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}#banner{padding:.75rem 1rem;background:#991b1b;color:#fff;font-size:.875rem;font-weight:600;border-radius:8px;max-width:420px;margin-bottom:1rem}#banner[hidden]{display:none}</style>

</head>

<body>

  <div id=banner hidden role=status aria-live=polite>

    You are offline — changes will sync when you reconnect.

  </div>

  <h1>Offline banner demo</h1>

  <p>Toggle offline mode in DevTools to see the banner.</p>



  <script>var banner=document.getElementById("banner");function syncBanner(){banner.hidden=navigator.onLine}window.addEventListener("offline",syncBanner),window.addEventListener("online",syncBanner),syncBanner()</script>

</body>

</html>

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HTML onoffline Attribute — Offline banner