HTML Code

<!DOCTYPE html>

<html>

<head>

  <title>CSS :required Basic Example</title>

  <style>

    body {

      font-family: system-ui, sans-serif;

      padding: 1.5rem;

    }



    label {

      display: block;

      font-weight: 600;

      font-size: 0.875rem;

      margin-bottom: 0.25rem;

      color: #334155;

    }



    input {

      display: block;

      width: 100%;

      max-width: 18rem;

      padding: 0.5rem 0.75rem;

      margin-bottom: 1rem;

      border-radius: 0.4rem;

      font-size: 0.875rem;

      box-sizing: border-box;

    }



    :required {

      border: 2px solid #dc2626;

      background: #fef2f2;

    }



    :optional {

      border: 2px solid #cbd5e1;

      background: #fff;

    }

  </style>

</head>

<body>

  <form>

    <label for="email">Email (required)</label>

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



    <label for="phone">Phone (optional)</label>

    <input type="tel" id="phone">

  </form>

</body>

</html>

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CSS :required Selector - Basic Required/Optional