High Performance Web Sites

More Than Just Images

Using a far future Expires header on images is fairly common, but this best practice should not be limited to images. A far future Expires header should be included on any component that changes infrequently, including scripts, stylesheets, and Flash components. Typically, an HTML document won't have a future Expires header because it contains dynamic content that is updated on each user request.

In the ideal situation, all the components in a page would have a far future Expires header, and subsequent page views would make just a single HTTP request for the HTML document. When all of the document's components are read from the browser's cache, the response time is cut by 50% or more.

I surveyed 10 top Internet sites in the U.S and recorded how many of the images, scripts, and stylesheets had an Expires or a Cache-Control max-age header set at least 30 days in the future. As shown in Table 5-2, the news isn't good. Three types of components are tallied: images, stylesheets, and scripts. Table 5-2 shows the number of components that are cacheable for at least 30 days out of the total number of components of each type. Let's see to what extent these sites employ the practice of making their components cacheable:

• Five sites make a majority of their images cacheable for 30 days or more.

• Four sites make a majority of their stylesheets cacheable for 30 days or more.

• Two sites make a majority of their scripts cacheable for 30 days or more.

Table 5-2. Components with ...