HTML Code

<!DOCTYPE html>
<html>
<head>
  <title>CSS :invalid Delayed 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: 0.35rem;
      color: #334155;
    }

    input {
      width: 100%;
      max-width: 18rem;
      padding: 0.5rem 0.75rem;
      border: 2px solid #cbd5e1;
      border-radius: 0.4rem;
      font-size: 0.875rem;
    }

    input:invalid:not(:placeholder-shown) {
      border-color: #dc2626;
      background: #fef2f2;
    }

    input:valid {
      border-color: #16a34a;
      background: #f0fdf4;
    }
  </style>
</head>
<body>
  <label for="username">Username (required)</label>
  <input type="text" id="username" required placeholder="Enter username" minlength="3">
  <p><small>Empty field stays neutral; errors appear after you type.</small></p>
</body>
</html>

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CSS :invalid Selector - Delayed Errors