They are probably more interested in CSS over features like a spell-checker because worst case you can copy and paste to/from an application with a spell-checker for the same end results (a spell-checked post).
The CSS cannot be handled on a user side with any reasonable ammount of effort (I suppose you could set up a proxy to parse PM and rewrite the tags, but that seems a bit overkill to change a font color on new replies.)

In reply to Re^2: Automatic spell checking when previewing by Ven'Tatsu
in thread Automatic spell checking when previewing by gri6507

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