User blog comment:DaNASCAT/Update Your CSS and JavaScript To Speed Up Your Wiki/@comment-4674838-20120823031400

This was stated earlier in the comments by Dinoguy1000, but I agree that using JavaScript to load critical CSS files is a bad idea.

With most JavaScript features, things are usually OK when it fails to load, the page is still readable. Graceful degradation can also be used.

But if you're using JavaScript to load CSS and the JavaScript fails / the user has JavaScript disabled in their browser / etc, they could (potentially) be seeing a completely unreadable page. Granted, this is an edge case, but CSS may need to perform in these cases.