If I put the following content in an HtmlTextBox, when I open the preview mode of the designer or look at it in the Html5 Report Viewer I get annoying extra gaps:

<p>Comment</p><p>Comment</p><ol><li>List</li><li>List</li><li>List</li></ol><p><br></p><p>Comment</p><p>Comment</p>

I tried to replace these <p>-tags with <div>-tags:

<div>Comment</div><div>Comment</div><ol><li>List</li><li>List</li><li>List</li></ol><div><br></div><div>Comment</div><div>Comment</div>

It helps. But there are some other tags that cannot be replaced (tags of ordered and unordered lists for example). And these gaps are still rendered.