Edit in JSFiddle

<table>
  <thead>
    <tr>
      <th>この行に</th>
      <th>ヒット</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>この行に</td>
      <td>ヒット</td>
    </tr>
    <tr>
      <td>hoge</td>
      <td>fuga</td>
    </tr>
  </tbody>
</table>

table tr:first-of-type{
  background-color: red;
}