As HtmlTextBox item supports only a limited number of HTML tags and CSS attributes, it cannot be used for rendering a richly-formatted text. The solution can be to use a WebBrowser control (https://msdn.microsoft.com/en-us/library/system.windows.forms.webbrowser.aspx) in a user function to render HTML to a bitmap and display it in a PictureBox item as described in this KB article: http://www.telerik.com/support/kb/reporting/details/how-to-render-any-html-tags-in-reports

Let us know if you think providing such functionality out-of-the-box would be beneficial for your reports.

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