HTML Code

<!DOCTYPE html>
<html>
<head>
  <title>CSS :default vs :checked Example</title>
  <style>
    fieldset {
      border: 1px solid #e2e8f0;
      border-radius: 8px;
      padding: 1rem;
      margin-bottom: 1rem;
    }

    legend {
      font-weight: 700;
      color: #1e293b;
      padding: 0 0.35rem;
    }

  /* Default option from HTML markup */
    input[type="radio"]:default {
      accent-color: #2563eb;
    }

    input[type="radio"]:default + label {
      color: #1d4ed8;
    }

  /* Currently selected (changes when user clicks) */
    input[type="radio"]:checked + label {
      text-decoration: underline;
    }
  </style>
</head>
<body>
  <fieldset>
    <legend>Plan (Email is default)</legend>

    <input type="radio" id="plan-a" name="plan" value="email" checked>
    <label for="plan-a">Email</label><br>

    <input type="radio" id="plan-b" name="plan" value="phone">
    <label for="plan-b">Phone</label>
  </fieldset>

  <p><small>:default highlights Email. :checked underlines whichever option is selected now.</small></p>
</body>
</html>

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CSS :default vs :checked Selector