HTML Code

<!DOCTYPE html>
<html>
<head>
  <title>CSS :valid Signup Example</title>
  <style>
    body {
      font-family: system-ui, sans-serif;
      padding: 1.5rem;
      background: #f8fafc;
    }

    .signup-form {
      max-width: 20rem;
      padding: 1.25rem;
      background: #fff;
      border: 1px solid #e2e8f0;
      border-radius: 0.65rem;
    }

    .signup-form h2 {
      margin: 0 0 1rem;
      font-size: 1.125rem;
      color: #1e293b;
    }

    .signup-form label {
      display: block;
      margin-bottom: 0.25rem;
      font-size: 0.8125rem;
      font-weight: 600;
    }

    .signup-form input {
      display: block;
      width: 100%;
      margin-bottom: 0.85rem;
      padding: 0.5rem 0.75rem;
      border: 2px solid #cbd5e1;
      border-radius: 0.4rem;
      box-sizing: border-box;
    }

    .signup-form input:valid {
      border-color: #16a34a;
      background: #f0fdf4;
    }

    .signup-form input:invalid {
      border-color: #dc2626;
      background: #fef2f2;
    }

    button {
      width: 100%;
      padding: 0.55rem;
      background: #2563eb;
      color: #fff;
      border: none;
      border-radius: 0.4rem;
      font-weight: 600;
      cursor: pointer;
    }
  </style>
</head>
<body>
  <form class="signup-form">
    <h2>Create account</h2>
    <label for="name">Full name</label>
    <input type="text" id="name" required placeholder="Your name">
    <label for="email">Email</label>
    <input type="email" id="email" required placeholder="you@example.com">
    <button type="submit">Sign up</button>
  </form>
</body>
</html>

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CSS :valid Selector - Signup Form