HTML Code

<!DOCTYPE html>

<html lang="en">

<head>

  <meta charset="utf-8">

  <title>focusout validation</title>

  <style>

    body { font-family: sans-serif; padding: 16px; max-width: 420px; }

    form { display: flex; flex-direction: column; gap: 12px; }

    label { font-weight: 600; }

    input { padding: 10px 12px; border: 2px solid #cbd5e1; border-radius: 6px; font-size: 16px; }

    input.required-missing { border-color: #dc2626; background: #fef2f2; }

    #hint { color: #64748b; font-size: 14px; margin-top: 8px; }

  </style>

</head>

<body>

  <form id="signup-form">

    <div>

      <label for="first">First name</label><br>

      <input type="text" id="first" name="first">

    </div>

    <div>

      <label for="last">Last name</label><br>

      <input type="text" id="last" name="last">

    </div>

    <div>

      <label for="email">Email</label><br>

      <input type="email" id="email" name="email">

    </div>

  </form>

  <p id="hint">Tab out of empty fields — .required-missing marks them.</p>

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

  <script>

    $("#signup-form").on("focusout", "input", function() {

      var $field = $(this);

      var empty = $field.val().trim().length === 0;

      $field.toggleClass("required-missing", empty);

    });

  </script>

</body>

</html>

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jQuery focusout event — form validation (Try It)