One point on the web performance list is: “Concatinate and minify all your css into one file”. However in IE (version lower than 10), you have an internal limit regarding the stylesheet file.
These are the facts:
- A sheet may contain up to 4095 rules
- A sheet may @import up to 31 sheets
- @import nesting supports up to 4 levels deep
The official statement on the MSDN blog here: http://blogs.msdn.com/b/ieinternals/archive/2011/05/14/internet-explorer-stylesheet-rule-selector-import-sheet-limit-maximum.aspx
If you eventually get over these marks (and with an optimized generated css file, it is more likely) , you have to split your stylesheets into multiple files (in Production).
For example, github.com currently uses two css files for its normal website, named github.css and github2.css.