All major browsers (except Internet Explorer) offer their own built-in spellcheckers that perform inline spell checking out-of-the box in the editable elements: textareas, editable iframe and div elements. The content area of RadEditor is editable IFRAME( or DIV if ContentAreaMode is set to DIV) and the required spell as you type feature is offered in Firefox, Chrome, Safari and Opera. It is very likely that the inline spellchecking will be also implemented by IE developers in some future versions of Internet Explorer.
At present, we do not plan to provide spell as you type
I also created a public PITS item for you feature request. If many users request it we will definitely consider its implementation. Here you can find the PITS Issue: Public URL
Your clients could use the inline AJAX spellchecker of RadEditor which you can test in this live demo: AJAX Spell Checker
or the built-in spell as you type spellcheckers of Firefox, Chrome, Safari and Opera.
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