HTML Code

<!DOCTYPE html>

<html lang="en">

<head>

  <meta charset="utf-8">

  <title>input required selector</title>

  <style>

    body { font-family: sans-serif; padding: 16px; max-width: 24rem; }

    label { display: block; margin: 10px 0; }

    input { width: 100%; padding: 8px; box-sizing: border-box; border: 1px solid #cbd5e1; border-radius: 6px; }

  </style>

</head>

<body>

  <label>Name <input type="text" required></label>

  <label>Email <input type="email" required></label>

  <label>Phone (optional) <input type="tel"></label>

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

  <script>

    $("input[required]").css({ borderLeft: "4px solid #dc2626" });

  </script>

</body>

</html>

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input[required] — required fields (Try It)