From WHATWG Wiki

The "logout" Link Relation

The "logout" link relation is used only within a <link> element to reference a resource that logs out the current "user." The href attribute must point to the same origin as the current document.

The user agent MUST NOT request the resource, except as follows:

• The title attribute is used as a "grouping" identifier -- all open documents within the same domain that have the same title attribute value are considered in the same "group."

• Upon closing all documents in the same group, the URI specified by the href attribute of the last window is sent a POST request with an empty body by the user agent.

An example:

  <link rel="logout" title="groupA" href="http://domain.tld/logout?param=value" />