HTML Code

<!DOCTYPE html>

<html lang=en>

<head>

  <meta charset=UTF-8>

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

  <title>oninvalid — required field</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}label{display:block;font-size:.8125rem;font-weight:500;margin-bottom:.35rem}input[type=text]{display:block;width:100%;max-width:280px;padding:.5rem .75rem;border:1px solid #cbd5e1;border-radius:8px;font-size:.875rem;margin-bottom:.35rem}.error{font-size:.8125rem;color:#b91c1c;min-height:1.25rem;margin-bottom:.75rem}button,input[type=submit]{padding:.45rem .85rem;border:1px solid #2563eb;border-radius:8px;background:#2563eb;color:#fff;font-size:.8125rem;cursor:pointer}</style>

</head>

<body>

  <h1>Required username</h1>

  <p>Submit with an empty field to see the custom invalid message.</p>

  <form id=signup>

    <label for=username>Username</label>

    <input id=username name=username required oninvalid='this.setCustomValidity("Username is required.")' oninput='this.setCustomValidity("")'>

    <p class=error id=err aria-live=polite></p>

    <input type=submit value=Submit>

  </form>



  <script>document.getElementById("username").addEventListener("invalid",(function(){document.getElementById("err").textContent="Username is required."})),document.getElementById("username").addEventListener("input",(function(){document.getElementById("err").textContent=""}))</script>

</body>

</html>

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HTML oninvalid Attribute — Required username