Add the support for 'border-collapse: collapse;' CSS in HTML services.

[ylussaud]

In the following example borders inside of cells should not be rendered:

<table style="border: currentColor; border-image: none; border-collapse: collapse;">
         <tbody>
                  <tr>
                          <td style="padding: 0cm 7px; border: 1px solid black; border-image: none; width: 205px;" valign="top">
                          <p style="margin-bottom: 13px;"><font color="#000000"><font face="Calibri"><font size="3">Column 1</font></font></font></p>
                          </td>
                          <td style="border-width: 1px 1px 1px medium; border-style: solid solid solid none; border-color: black black black currentColor; padding: 0cm 7px; width: 205px;" valign="top">
                          <p style="margin-bottom: 13px;"><font color="#000000"><font face="Calibri"><font size="3">Column 2</font></font></font></p>
                          </td>
                          <td style="border-width: 1px 1px 1px medium; border-style: solid solid solid none; border-color: black black black currentColor; padding: 0cm 7px; width: 205px;" valign="top">
                          <p style="margin-bottom: 13px;"><font color="#000000"><font face="Calibri"><font size="3">Column 2</font></font></font></p>
                          </td>
                  </tr>
                  <tr>
                          <td style="border-width: medium 1px 1px; border-style: none solid solid; border-color: currentColor black black; padding: 0cm 7px; width: 205px;" valign="top">
                          <p style="margin-bottom: 13px;"><font color="#000000"><font face="Calibri"><font size="3">Value a</font></font></font></p>
                          </td>
                          <td style="border-width: medium 1px 1px medium; border-style: none solid solid none; border-color: currentColor black black currentColor; padding: 0cm 7px; width: 205px;" valign="top">
                          <p style="margin-bottom: 13px;"><font color="#000000"><font face="Calibri"><font size="3">Value b</font></font></font></p>
                          </td>
                          <td style="border-width: medium 1px 1px medium; border-style: none solid solid none; border-color: currentColor black black currentColor; padding: 0cm 7px; width: 205px;" valign="top">
                          <p style="margin-bottom: 13px;"><font color="#000000"><font face="Calibri"><font size="3">Value c</font></font></font></p>
                          </td>
                  </tr>
         </tbody>
</table>

[ylussaud]

That's the current behavior of M2Doc. Borders data is missing from MCell to implement border-collapse: separate;.