You can use multiple <tbody> tags in the same table, to more easily style groups of data, like this:
thead th { width: 100px; border-bottom: solid 1px #ddd; font-weight: bold; }
tbody:nth-child(odd) { background: #f5f5f5; border: solid 1px #ddd; }
tbody:nth-child(even) { background: #e5e5e5; border: solid 1px #ddd; }
<table>
<thead>
<tr><th>Customer</th><th>Order</th><th>Month</th></tr>
</thead>
<tbody>
<tr><td>Customer 1</td><td>#1</td><td>January</td></tr>
<tr><td>Customer 1</td><td>#2</td><td>April</td></tr>
<tr><td>Customer 1</td><td>#3</td><td>March</td></tr>
</tbody>
<tbody>
<tr><td>Customer 2</td><td>#1</td><td>January</td></tr>
<tr><td>Customer 2</td><td>#2</td><td>April</td></tr>
<tr><td>Customer 2</td><td>#3</td><td>March</td></tr>
</tbody>
<tbody>
<tr><td>Customer 3</td><td>#1</td><td>January</td></tr>
<tr><td>Customer 3</td><td>#2</td><td>April</td></tr>
<tr><td>Customer 3</td><td>#3</td><td>March</td></tr>
</tbody>
</table>
This will only work in newer browsers, you can use the grouping for JavaScript, etc. It's a convenient way to visually group the rows to make the data much more readable.