Rated 3 out of 5 stars

Does not support the Cyrillic alphabet.Example of use:
button [label = "Открыть папку"] {display: none;}
In the subsequent editing spoil the "Открыть папку".

This review is for a previous version of the add-on (0.6.20130108.1-signed.1-signed). 

Time to learn CSS

CSSsir works just with CSS properties and NOT with the html tags or CSS selectors (except TagCleaner tool). Selecors tab in Bonus contains only CSS tips partly taken from http://www.w3.org/TR/CSS2/selector.html

Any question about work of CSS selectors you can ask World Wide Web Consortium (W3C)

I strongly advise you not to use cyrillic alphabet in CSS selectors. Try to use standart id or class selectors in latin.

P.S. To solve your problem try charset "utf-8" in CSS files and
<meta http-equiv='Content-Type' content='text/html; charset=utf-8'>