Implementing the full HTML specification is outside the scope of the rich text formatted texts in the control. This will simply be an overkill (we have implemented almost everything found in HTML in our NOV Rich Text Control) and be too heavy for processing. The newest SP of the control exposes a paragraph attribute that allows you to specify additional line spacing - for example:
<p linespacing='10pt'>line 1<br/>line 2</p>
Hope this helps - let us know if you meet any problems.
Nevron Support Team