Unfortunately the HTML that HtmlFormatProvider exports is not fully compatible with the HTML that the HtmlTextBox report item recognizes. The list of supported tags in HtmlTextBox can be found here
and the customization possibilities of the HtmlFormatProvider are described in this article
. Maximum compatibility is achieved by setting the DocumentExportLevel to Fragment ad StylesExportMode to Inline, but there are other issues that cannot be worked around, namely the recognition of Bold and Underlined parts of the document.
If you would be showing the exported document in a report, we can send you the source code of RadRichTextBox and HtmlFormatProvider, so that you can create a custom HTML format provider that will be compatible with HtmlTextBox. In that case, please open a support ticket requesting the source code.
We will try to make HtmlTextBox and RadRichTextBox compatible as soon as possible, but the time frame is still not certain. We've created a PITS issue, which you can track and vote for in order to increase its priority. Here you can find the PITS Issue: Public URL
All the best,
the Telerik team
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