Why would you use JavaScript to request the friends list JSON and then use the DOM to insert a stylesheet? Just CSS would be fine if you're changing the color of friends' usernames and not doing anything that needs JavaScript.
The idea is to remove the part of loading friends from the page loading. By delay the friends JS to run only after the page is loaded, the page loads faster, and can be loaded from cache. The cached page doesn't know your friends. You then have the JS load a page with friends as a separate request. This shifts the request from the loading of the page to after the page is loaded, meaning the page gets loaded faster, but missing the small feature of displaying friends.
