“Have you ever watched your status bar while you wait for a page to load and wondered why several files seem to be downloaded before you see anything at all on your screen? Eventually the page content displays, and then the images are slotted in. The files that keep you waiting are generally the CSS and Javascript files linked to from the ‘head’ section of the HTML document. Because these files determine how the page will be displayed, rendering is delayed until they are completely downloaded.”
Link:
http://www.sitepoint.com/blogs/2007/04/10/faster-page-loads-bundle-your-css-and-javascript/
