Google+ Sign-In button allows users to authenticate on websites using their Google+ account.
Use your Facebook account to sign into other sites and see what your friends are doing across the web.
Google+ platform websites integration using the Google+ platform tag.
Open graph description entity tag
The http-equiv attribute provides an HTTP header for the information/value of the content attribute. The http-equiv attribute can be used to simulate an HTTP response header.
A canonical link element is an HTML element that helps webmasters prevent duplicate content issues by specifying the "canonical", or "preferred".