This and that you cannot preview comments are both "features" of the original stack-exchange software. Remember that the original software did not support mathematics using LaTeX expressions: that was a customization for MathOverflow. So for the most part there was no need to "check the layout" of the comments.
That said: the usual blurb about how the actual software is outside of our control should be inserted here.
BTW, because of the lack of previews in comments, Will Jagy has the habit of doing his comments in a separate editor, test compile it, before copying over to MO. I think he won't mind if you e-mail to ask about his workflow.
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