At http://www.mediawiki.org/w/index.php?title=ResourceLoader/Features&oldid... it is written:
It starts by performing a quick sanity check that bails out if the current browser is not supported. This saves bandwidth as well as preventing broken interfaces, basically leaving the user with an untouched page with the natural non-javascript fallback behavior. Browsers such as Internet Explorer 5 and early versions of Mozilla fall in this category.
While I see that it works, I can't find where exactly in the code is this sanity check located. Any pointers?