You won't be able to fix it on your end by editing the post and correcting the CSS -- the conversion is being done at the time the entry is loaded, not at the time the entry is saved. We have to figure out what to do about it on our end.

In addition to breaking all discussions of CSS going forward and backward, this does the same to all discussions of perl code, where both "at-sign foo" and "backslash at-sign foo" are very common idioms that should remain untouched. Using "don't autoformat" isn't really a solution, because it also breaks (IIRC) the "line break to <br />" conversions. While I can see the attraction of "at-sign exampleuser" for some people going forward the only 2 backward-compatible solutions that I see (but I may be missing some) are:
- make the at-sign expansion opt-in (making it opt-out won't work, because it still breaks backward compatibility)
- keep at-sign expansion for markdown and markdown only (which is another way of making it opt-in, I guess)