Hello,
We have tested it and it's working fine:
$html = '
<style>
  table {border: 0;border-collapse: collapse; font-size: 14px !important;}
  td {border: 1px solid black;font-size: 14px !important;}
</style>
<table style="font-size: 14px">
<tbody>
<tr>
  <td>
    This is a text
  </td>
  <td>
    Text B
  </td>
</tr>
<tr>
  <td>
    Text C
  </td>
  <td>
    Text D
  </td>
</tr>
</tbody>
</table>
';
$docx->embedHTML($html);
If we change the font size to 18, the font size is higher when we open the DOCX:
$docx = new CreateDocx();
$html = '
<style>
  table {border: 0;border-collapse: collapse; font-size: 18px !important;}
  td {border: 1px solid black;font-size: 18px !important;}
</style>
<table style="font-size: 18px">
<tbody>
<tr>
  <td>
    This is a text
  </td>
  <td>
    Text B
  </td>
</tr>
<tr>
  <td>
    Text C
  </td>
  <td>
    Text D
  </td>
</tr>
</tbody>
</table>
';
$docx->embedHTML($html);
We recommend you to check if Tidy for PHP is installed and enabled, test the previous scripts and check your script.
Regards.