HTML Code

<!DOCTYPE html>
<html lang=en>
<head>
  <meta charset=UTF-8>
  <meta name=viewport content="width=device-width,initial-scale=1">
  <title>showModal 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}.open-btn{padding:.5rem 1rem;border:none;border-radius:6px;background:#2563eb;color:#fff;cursor:pointer;font:inherit}dialog{border:none;border-radius:12px;padding:1.5rem;max-width:340px;box-shadow:0 20px 40px rgba(15,23,42,.2)}dialog::backdrop{background:rgba(15,23,42,.45)}dialog h2{margin:0 0 .5rem;font-size:1.1rem;color:#0f172a}dialog p{margin:0 0 1rem;color:#475569}.actions{display:flex;gap:.5rem;justify-content:flex-end}.actions button{padding:.45rem .9rem;border:none;border-radius:6px;cursor:pointer;font:inherit}.cancel{background:#e2e8f0;color:#334155}.confirm{background:#2563eb;color:#fff}</style>
</head>
<body>
  <h1>Modal dialog</h1>
  <button type=button class=open-btn id=openBtn>Open modal</button>

  <dialog id=welcomeDialog>
    <h2>Welcome!</h2>
    <p>Please confirm you want to continue.</p>
    <div class=actions>
      <form method=dialog>
        <button type=submit class=cancel>Cancel</button>
      </form>
      <form method=dialog>
        <button type=submit class=confirm value=confirmed>Confirm</button>
      </form>
    </div>
  </dialog>

  <script>const dialog=document.getElementById("welcomeDialog");document.getElementById("openBtn").addEventListener("click",()=>{dialog.showModal()}),dialog.addEventListener("close",()=>{"confirmed"===dialog.returnValue&&alert("Confirmed!")})</script>
</body>
</html>

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Modal dialog with showModal()