See bug 984880 comment 71. With naive as-authored editing, it is possible to get the style sheet into a bad state if one enters invalid property values in the rule view. The rule view property editor should reject invalid edits and notify the user.

I think bug 1196896 will provide some of the infrastructure needed for this. The other bit is either using the appropriate DOMUtils method, if it does the right thing for unpaired braces; or otherwise maybe writing a simple parser.

It turns out that this is completely handled by some changes to the rewriter in bug 1195356, and there's nothing particular to do here.