Currently unknown/unsupported elements are skipped, which potentially loses text content. Consider adding setting in HtmlImportOptions allowing the phrasing content (text) of these elements to be imported. Sample HTML: <html> <body> <test-test>test-test</test-test> <body/> </html> Browsers visualize it as "test-test", while RadRichTextBox doesn't import anything. MS Word imports such elements as inline elements (just like spans).