Then right-click in an HTML file (and only in an HTML file) and choose "Check Spelling". Using a file that must be "c:/jazzy/english.0" (such as this one), the IDE will then perform a spell check and the result with be displayed in a viewer (accessible under the Window menu, by choosing "View Spell Errors"):
Making the actual spelling corrections and extending the dictionary can only be done manually. (And, as stated above, the dictionary must have the specified name and must be in the specified location, because I haven't added the Options window extension yet.) If someone wants to extend this functionality (based on the open source Jazzy library, which I blogged about quite a lot a few weeks ago), please let me know and I'll send you the sources.