4.12.5.9 Link type "prefetch"
The prefetch keyword may be used with link, a, and area elements. This keyword creates an external resource link.
The prefetch keyword indicates that preemptively fetching and caching the specified resource is likely to be beneficial, as it is highly likely that the user will require this resource.
There is no default type for resources given by the prefetch keyword.
Resource is not a valid keyword here, and prefetch seems to fit.
http://www.w3.org/html/wg/drafts/html/master/links.html#link-type-prefetch