HTML Code

<!DOCTYPE html>
<html>
<head>
  <title>CSS :invalid 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: 2px solid #cbd5e1;
      border-radius: 0.4rem;
      font-size: 0.875rem;
      box-sizing: border-box;
    }

    :invalid {
      border-color: #dc2626;
      background: #fef2f2;
    }

    :valid {
      border-color: #16a34a;
      background: #f0fdf4;
    }
  </style>
</head>
<body>
  <form>
    <label for="email">Email:</label>
    <input type="email" id="email" name="email" placeholder="Enter your email" required>

    <label for="age">Age (18–65):</label>
    <input type="number" id="age" name="age" min="18" max="65" placeholder="Enter your age" required>
  </form>
  <p><small>Try invalid email or age outside 18–65.</small></p>
</body>
</html>

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CSS :invalid Selector - Form Validation