Hi,
the current version of the RTF Filler component available on the Add-Ons Store supports a subset of the available XHTML tags.
The subset of the available XHTML tags that can be used to fill the RTF template is reported below:
Paragraph <p>
Ordered list <ol>
Unordered list <ul>
List item <li>
Table <table>
Row <tr>
Default cell <td>
Header cell <th>
Bold <b>
Strong <strong>
Italic <i>
Emphasize <em>
Underline <u>
Strike <strike>
Subscript <sub>
Superscript <sup>
If you want to obtain your result you can try to extend the component and implement this feature.
For more detail about the structure of a RTF document you can refer to the Rich Text Format (RTF) Specifications available in Internet.
Moreover you can refer to these guides:
https://my.webratio.com/learn/learningobject/custom-unit-guide
https://my.webratio.com/learn/learningobject/how-to-debug-the-code-of-a-custom-unit-inside-the-web-application