Caching. If a user revisits a web page after only a short interval, the page data may not need to be re-obtained from the source web server. Almost all web browsers cache recently-obtained data, usually on the local hard drive. HTTP requests sent by a browser will usually only ask for data that has changed since the last download. If the locally-cached data is still current, it will be reused.
Caching helps reduce the amount of web traffic on the Internet. The decision about expiration can be made independently for each downloaded file, whether image, stylesheet, JavaScript, HTML, or whatever other content the site may provide. Thus even on sites with highly dynamic content, many of the basic resources may only need to be refreshed once every few sessions. Web site designers may find it worthwhile to collate shared resources such as CSS data and JavaScript into a few site-wide files so that they can be cached efficiently. This helps reduce page download times and lowers demands on the web server.