Internet Explorer 9 has a bug which means it can't process more than 31 top-level stylesheets. (This actually affects 7 and 8 as well, but people are really good about updating Internet Explorer, aren't they? What's that? Oh)
The problem comes from the way IE stores CSS rules. Each rule is referenced as a 32 bit number. The first 5 bits are the ID of the top level stylesheet the rule came from, and the last 12 bits are the rule. The 15 bits in the middle are @import references. You can chain @imports three levels down (so 5 bits each again), but only from those first 31 stylesheets.
document.styleSheets.cssText += ourCss;
The browser now processes the CSS, without busting its 31 stylesheet limit. You can't use this to insert an @import however as that isn't strictly cssText, it doesn't get reprocessed.