Are there any restrictions on what is prefetched?

[hassen1]

Yes, only http:// URLs can be prefetched (https:// URLs are never prefetched for security reasons). Other protocols (such as FTP) do not provide rich enough support for client side caching. In addition to this restriction, URLs with a query string are not prefetched. This is done because such URLs often result in documents that cannot be reused out of the browser's cache, so prefetching them often has little benefit. We found that some existing sites utilize the <link rel="next"> tag with URLs containing query strings to reference the next document in a series of documents. Bugzilla is an example of such a site that does this, and it turns out that the Bugzilla bug reports are not cachable, so prefetching these URLs would nearly double the load on poor Bugzilla! It's easy to imagine other sites being designed like Bugzilla, so we explicitly do not prefetch URLs with query strings. (It might make sense to allow prefetching of these documents when the rel=prefetch relation type is specified, since this should not appear in any existing content.) There are no other restrictions on the URLs that are prefetched.