HTML Code

<!DOCTYPE html>
<html>
<head>
  <title>CSS :valid Basic Example</title>
  <style>
    :valid {
      border: 2px solid #16a34a;
      background-color: #f0fdf4;
    }

    :invalid {
      border: 2px solid #dc2626;
      background-color: #fef2f2;
    }

    input {
      display: block;
      width: 100%;
      margin-bottom: 0.75rem;
      padding: 0.5rem;
      border-radius: 0.4rem;
      font-family: system-ui, sans-serif;
      box-sizing: border-box;
    }

    label {
      display: block;
      margin-bottom: 0.25rem;
      font-size: 0.875rem;
      font-weight: 600;
    }

    body {
      font-family: system-ui, sans-serif;
      padding: 1.5rem;
      max-width: 22rem;
    }
  </style>
</head>
<body>
  <form>
    <label for="email">Email:</label>
    <input type="email" id="email" required placeholder="you@example.com">

    <label for="age">Age (18–60):</label>
    <input type="number" id="age" min="18" max="60" required placeholder="25">

    <button type="submit">Submit</button>
  </form>
</body>
</html>

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CSS :valid Selector - Valid and Invalid Form