Currently you cannot preview your comment in AUR. There is a message that markdown is partly supported. Sometimes you create a comment, but it is rendered unexpectedly for you. For example, when you forget to add some white space near code blocks.

After you have created such comment, you can edit it and see if your edit is correct. But still, you do it blindly with edit-save-observe cycle.

It would be nice to have ability to preview rendering before submitting a comment.