<!DOCTYPE html>
<html>
<head>
<style>
table {
border-collapse: collapse;
width: 100%;
border: 1px solid #ddd;
}
th, td {
padding: 8px;
text-align: left;
border-bottom: 1px solid #ddd;
}
th {
background-color: #f2f2f2;
}
</style>
</head>
<body>
<h2>Nested HTML Tables Example</h2>
<table>
<tr>
<th>Main Table Header 1</th>
<th>Main Table Header 2</th>
<th>Main Table Header 3</th>
</tr>
<tr>
<td>
<table>
<tr>
<th>Nested Table Header 1</th>
<th>Nested Table Header 2</th>
</tr>
<tr>
<td>Nested Table Data 1</td>
<td>Nested Table Data 2</td>
</tr>
</table>
</td>
<td>Main Table Data 2</td>
<td>Main Table Data 3</td>
</tr>
<tr>
<td>Main Table Data 1</td>
<td>Main Table Data 2</td>
<td>Main Table Data 3</td>
</tr>
</table>
</body>
</html>