I dear say that everyone who's used the HTML Editor control will agree that it is not really adequate for serious use. So, I propose several enhancements to the API:

1) Make HtmlText an ObservableValue - CRUCIAL.
2) Allow toolbar customization. At minimum, this should include disabling/enabling or hiding/showing available controls.
3) Provide a mechanism to limit the number (and type) of available fonts. This is essential for web content production (web-safe fonts are still considered a good design practice, but also for the theme-specific fonts), and when preparing content for external components, like JasperReports.
4) Allow selecting default font from code.