However, there are a number of other factors that affect page rendering times—that is, the time between pressing the Enter key or clicking a link, and the page being fully displayed in the browser. These factors include network latency, server speeds, number of requests required (each image, external style sheet, and JavaScript fi le is a separate request), and the complexity of the CSS and any JavaScript that renders content to the screen. Some of these things are within your control, and some not. Yahoo!’s Yslow is a Firefox add-on that extends Firebug’s capability and can help you further understand what is affecting your pages’ performance. If you run a large Web site, I recommend you run your pages through Firebug and look at the Yslow tab also. Discussion around site performance and links to these and other tools can be found at http://davidherron.com/book-page/190-measuring-website-speed.