Link prefetching

Link prefetching is a method for web pages to declare hints allowing web browsers to pre-load various external resources to speed up loading and rendering of the page. The resources can include JavaScript, CSS, image, audio, video or web font files as well as DNS names and connections.

Below are various methods that enable different types of link prefetching in supported browsers using HTML5 markup language to display a given link:

The prefetch hints can be also sent in LinkHTTP headers, for example:

Two W3C standards discuss different forms of prefetch:

It is possible for implementations to prefetch links even when they are not specified as prefetch links.

An issue inherent to indiscriminate link prefetching involves the misuse of "safe" HTTP methods. The HTTP GET and HEAD requests are said to be "safe", i.e., a user agent that issues one of these requests should expect that the request results in no change on the recipient server. However, it's not uncommon for website operators to use these requests outside of this constraint. Plain hyperlinks (which almost universally result in GET requests) are often used to implement logout functionality and account verification, e.g., when a user completes an account creation form, and an automated service sends a verification e-mail to the user's given e-mail address. Similarly, it is entirely possible for a hosting service to provide a Web front end to manage files, including links that delete one or more files. Users who visit pages containing these types of links, (whilst using a browser which employs an indiscriminate link prefetcher), might find that they have been logged out or that their files have been deleted.

Additionally, there are a number of criticisms regarding the privacy and resource usage implications of link prefetching:

In the case of mobile devices or for users with a limited bandwidth allowance, prefetching may result in an unnecessary costly drain on limited bandwidth.

In the case of prerendering, Google warns that improper use may result in the aforementioned increased bandwidth usage, slower loading of other links, and slightly stale content.

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