Link type "sidebar"
The sidebar keyword may be used with link, a, and area elements. This keyword creates a hyperlink.
The sidebar keyword indicates that the referenced document, if retrieved, is intended to be shown in a secondary browsing context (if possible), instead of in the current browsing context.
A hyperlink with the sidebar keyword specified is a sidebar hyperlink as defined in the HTML standard: http://www.whatwg.org/specs/web-apps/current-work/#rel-sidebar-hyperlink