<table>
	<tr>
		<th>見出し1</th>
		<td colspan="3">内容</td>
	</tr>
	<tr>
		<th>見出し2</th>
		<td>内容</td>
		<th>見出し3</th>
		<td>内容</td>
	</tr>
</table>

* {
	box-sizing: border-box;
}
body {
	min-height: 100vh;
	margin: 0;
	padding: 20px;
	display: flex;
	justify-content: center;
	align-items: center;
}
table {
    width: 100%;
    border-collapse: collapse;
}
table th,
table td {
    padding: 10px;
    border: 1px solid #ccc;
}
table th {
    background: #f5f5f5;
}

table {
	table-layout: fixed;
}
table th {
	width: 100px;
}