The Performance Golden Rule, as explained in Chapter 1, reveals that only 10–20% of the end user response time involves retrieving the requested HTML document. The remaining 80–90% of the time is spent making HTTP requests for all the components (images, scripts, stylesheets, Flash, etc.) referenced in the HTML document. Thus, a simple way to improve response time is to reduce the number of components, and, in turn, reduce the number of HTTP requests.
Suggesting the idea of removing components from the page often creates tension between performance and product design. In this chapter, I describe techniques for eliminating HTTP requests while avoiding the difficult tradeoff decisions between performance and design. These techniques include using image maps, CSS sprites, inline images, and combined scripts and stylesheets. Using these techniques reduces response times of the example pages by as much as 50%.
In its simplest form, a hyperlink associates the destination URL with some text. A prettier alternative is to associate the hyperlink with an image, for example in navbars and buttons. If you use multiple hyperlinked images in this way, image maps may be a way to reduce the number of HTTP requests without changing the page's look and feel. An image map allows you to associate multiple URLs with a single image. The destination URL is chosen based on where the user clicks on the image.
Figure 3-1 shows an example of five images used ...