As you haven't mentioned the HTML you're trying to import, please make sure it's valid XHTML in order to achieve best results.
As for the ImportHtml method, indeed it's correct as per the demo, however since the Q2 SP1 it can be shortened as:
However, regarding the method you're using to import the document, I'd recommend you to avoid using the SetText as it loses all the formatting. Rather, use this code in order to put a document inside your RadRichTextBox:
richtextbox.Document = ImportHtml(reader.ReadToEnd());
Let us know if you need any further assistance.
the Telerik team
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