Edit in JSFiddle

CSS
HTML
Result

table, th, td {border: 1px solid black;}

.ex1 {
  border-collapse: separate;
}

.ex2 {
  border-collapse: collapse;
}

<!DOCTYPE html>
<html lang="ko">
  <head></head>
  <body>
    <p>border-collapse: separate</p>
        <table class="ex1">
            <tr>
                <th>Firstname</th>
                <th>Lastname</th>
            </tr>
            <tr>
                <td>Peter</td>
                <td>Griffin</td>
            </tr>
            <tr>
                <td>Lois</td>
                <td>Griffin</td>
            </tr>
        </table>

        <p>border-collapse: collapse</p>
        <table class="ex2">
            <tr>
                <th>Firstname</th>
                <th>Lastname</th>
            </tr>
            <tr>
                <td>Peter</td>
                <td>Griffin</td>
            </tr>
            <tr>
                <td>Lois</td>
                <td>Griffin</td>
            </tr>
        </table>
  </body>
</html>