As mentioned in this post: http://wiki.languagetool.org/tips-and-tricks (I quoted below).
I tried downloading XXE and open rules.css file with the software.
I am supposed to see a WYSIWYG rule editor to make my own rule, however it seems I did a wrong way.
Please help me!
Using xxe for editing rules
It's possible to edit rules using XML Mind XML Editor (xxe) in a WYSYWIM (what you see is what you mean) mode. The program is available in a free version for many platforms (as it's developed in Java) though it's not open source. It offers spell-checking and validation, and we have developed special CSS files to make editing files easier. Put these in the rules directory.
Simply open the file in xxe, and it should display in a form input mode. It is recommended
that you uncheck Options > Preferences > Save > Add open lines. Set a high value in Max line length.