HTML Code

<!DOCTYPE html>

<html lang="en">

<head>

  <meta charset="utf-8">

  <title>form has error demo</title>

  <style>

    body { font-family: sans-serif; padding: 16px; max-width: 28rem; }

    form { padding: 12px; margin: 12px 0; border: 2px solid #e2e8f0; border-radius: 8px; }

    form.has-errors { border-color: #dc2626; background: #fef2f2; }

    label { display: block; margin: 8px 0; }

    input { width: 100%; padding: 8px; box-sizing: border-box; margin-top: 4px; }

    .error { border-color: #dc2626; }

  </style>

</head>

<body>

  <form id="signup">

    <label>Username <input class="error" type="text" value=""></label>

    <label>Email <input type="email" value="bad"></label>

  </form>

  <form id="contact">

    <label>Name <input type="text" value="Ada"></label>

    <label>Message <input type="text" value="Hi"></label>

  </form>

  <script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>

  <script>

    $("form:has(.error)").addClass("has-errors");

  </script>

</body>

</html>

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form:has(.error) — validation UI (Try It)