I can't apply a style to a snippet

If you can successfully apply bold to text in the regular paragraph above the table, perhaps you have a complex selector which specifically sets paragraphs inside table cells to normal weight.

I can't say exactly what might be set, because of the cascading nature of css, but look for something like:

td p {

    font-weight: normal;

}

In HTML Header Row is a specific tag. It looks like you simply mean the top row. That is not the same thing.

However you define the first row though, it should be possible to bold the content. I just created a table in a snippet, put text in the cells of the first row, selected the tr breadcrumb and then pressed CTRL B. Bold was applied to all the text. I was also able to apply bold to individual cells or words.

Try a new snippet to test.

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You say 'header row', but I only see <td> elements in your code. There are no <th> elements, which are used to indicate table header cells.

i.e. it should be

[...]

</colgroup>

<tbody>

  <tr>

     <th colspan="2" valign="top">

       <p>...</p>

     </th>

  </tr>

...

Most CSS stylesheets tend to already bold header rows by applying styles to th elements.