<table border="1">
    <tr>
        <th></th>
        <th>ヘッダー</th>
        <th>ヘッダー</th>
    </tr>
    <tr>
        <th>ヘッダー</th>
        <td>セル</td>
        <td>セル</td>
    </tr>
    <tr>
        <th>ヘッダー</th>
        <td>セル</td>
        <td>セル</td>
    </tr>
</table>
body{
    margin: 0;
}
table{
    width: 100vw;
    height: 100vh;
}

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