I am assuming you are trying to import an HTML file containing a hover selector (as in the provided code snippet) then to visualize it in RichTextEditor. If so, this feature is currently not supported and I logged a feature request in our backlog: RadRichTextEditor: HtmlFormatProvider add support for hover selector. You can cast your vote for the implementation as well as subscribe to the task by clicking the Follow button so you can receive notifications about status updates.
In appreciation for pointing this to our attention, I have added 500 Telerik points to your account.
I hope you find this information helpful. Please, let me know if there is anything else I can assist you with.
Regards, UI for WinForms Martin
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