Per https://bugzilla.wikimedia.org/show_bug.cgi?id=32183 ...
In r101860 I've removed most of the client-* classes from the <html> that were added based on user-agent sniffing. Sniffing UA strings is a fragile practice which should be strongly discouraged in favor of checking for the actual features or bugs that you need to address -- encouraging people to use styles like "client-firefox7" makes me feel a little ill inside. :)
The client-js / client-nojs classes remain; they are in fact a good example of use of feature detection (whether JS is enabled or not) which allows targeting display of UI elements specifically at that condition.
No extensions in SVN or core code appears to be using any of the other client-* classes; there could be some in user JS/CSS code -- if you've been using them or seen someone use them, you should see about fixing them up to make sure they don't break in unexpected browsers or on MediaWiki 1.19.
-- brion