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The <colgroup> element is used to group columns in an HTML table and apply styles to them collectively. It is typically used in conjunction with the <col> element to define the properties of multiple columns at once.

Here is an example of how <colgroup> can be used:

<table>
  <colgroup>
    <col style="background-color: lightblue;">
    <col style="background-color: lightgreen;">
    <col style="background-color: lightyellow;">
  </colgroup>
  <tr>
    <th>Column 1</th>
    <th>Column 2</th>
    <th>Column 3</th>
  </tr>
  <tr>
    <td>Data 1</td>
    <td>Data 2</td>
    <td>Data 3</td>
  </tr>
</table>

In this example, the <colgroup> element is used to group three <col> elements, each with a different background color. These styles will be applied to the respective columns in the table.

Note that the <colgroup> element must be placed immediately after the opening <table> tag, and it should contain one or more <col> elements. The <col> elements can have various attributes and styles to define the properties of the columns.