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22 Chapter 3 CHAPTER 3 Rule 3: Add an Expires Header 3 Fast response time is not your only consideration when designing web pages. If it were, then we'd all take Rule 1 to an extreme and place no images, scripts, or stylesheets in our pages. However, we all understand that images, scripts, and stylesheets can enhance the user experience, even if it means that the page will take longer to load. Rule 3, described in this chapter, shows how you can improve page performance by making sure these components are configured to maximize the browser's caching capabilities. Today's web pages include many components and that number continues to grow. A first-time visitor to your page may have to make several HTTP requests, but by using a future Expires header, you make those components cacheable. This avoids unnec- essary HTTP requests on subsequent page views. A future Expires header is most often used with images, but it should be used on all components, including scripts, stylesheets, and Flash. Most top web sites are not currently doing this. In this chap- ter, I point out these sites and show why their pages aren't as fast as they could be. Adding a future Expires header incurs some additional development costs, as described in the section "Revving Filenames." Expires Header Browsers (and proxies) use a cache to reduce the number of HTTP requests and decrease the size of HTTP responses, thus making web pages load faster. A web server uses the Expires header to tell the web client that it can use the current copy of a component until the specified time. The HTTP specification summarizes this header as "the date/time after which the response is considered stale." It is sent in the HTTP response. Expires: Thu, 15 Apr 2010 20:00:00 GMT