HTML Code

<!DOCTYPE html>
<html>
<head>
  <title>CSS :has() Form Validation</title>
  <style>
    body {
      font-family: system-ui, sans-serif;
      padding: 1.5rem;
    }

    form {
      max-width: 18rem;
      padding: 1rem;
      border: 2px solid #e2e8f0;
      border-radius: 0.5rem;
      background: #f8fafc;
    }

    form:has(:invalid) {
      border-color: #dc2626;
      background: #fef2f2;
    }

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

    input {
      display: block;
      width: 100%;
      padding: 0.5rem;
      margin-bottom: 0.75rem;
      border: 1px solid #cbd5e1;
      border-radius: 0.35rem;
      font-size: 0.875rem;
    }

    input:invalid {
      border-color: #dc2626;
    }
  </style>
</head>
<body>
  <form>
    <label for="email">Email (required)</label>
    <input type="email" id="email" required placeholder="you@example.com">
    <label for="name">Name</label>
    <input type="text" id="name" placeholder="Your name">
  </form>
  <p><small>Leave email empty to see the form highlight turn red.</small></p>
</body>
</html>

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CSS :has() Selector - Invalid Form Highlight