rel attribute.

 Values:
 
alternate: 	Links to an alternate version of the document (i.e. print page, translated or mirror)
author: 	Links to the author of the document
bookmark: 	Permanent URL used for bookmarking
help: 	Links to a help document
license: 	Links to copyright information for the document
next: 	The next document in a selection
nofollow: 	Links to an unendorsed document, like a paid link. ("nofollow" is used by Google, to specify that the Google search spider should not follow that link)
noreferrer: 	Specifies that the browser should not send a HTTP referer header if the user follows the hyperlink
prefetch: 	Specifies that the target document should be cached
prev: 	The previous document in a selection
search: 	Links to a search tool for the document
tag: 	A tag (keyword) for the current document