If I would have to make a bet, I would guess it won't be default.
The possibility of HTML editing is more ore less a byproduct of enabling Mail users to choose the editor component, for example KVimPart for VIM users or the above mentioned HTML enabled editor, however that will be implemented.
Yes, I am also confident that it won't be default. Seriously speaking it is a good thing that they are making it an option to write HTML mails since someone might find it useful to use HTML in mails, although I hope I never ever have to receive more HTML mails
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Kmail team already has made kmail so good that I have abandon pine (sob, it was nice knowing you pine). Damn you!
And now, where did you get that kvim example here, did you read my other post?