There is a bug with YUI compressor and media queries that requires inserting a comment in CSS as a workaround. This bug has been fixed in YUI compressor since at least 2.4.6 (in 2011) http://yuiblog.com/blog/2011/04/26/yui-compressor-2-4-6/
Fixing this means less work is needed to create stylesheets for BBIS. This is especially helpful if migrating an existing design into BBIS.
We are planning an upgrade to a different YUI compressor entirely. This is tentatively planned for SP11, but stay tuned for more details.