What is <link rel="image_src">
html, xhtml
Solution
The `rel` attribute specifies the type of the link, i.e. the kind of the relationship between the document and the linked resource. Usually just a few keywords, like `stylesheet` and `icon`, are used. Although many other keywords have been proposed and registered, most of them are write-only: they are meant to express something, but nobody cares (no software uses the information).
The extension mechanisms of HTML5 include, in the description of link types, a somewhat obscure mechanism that allows, in theory, anyone register his favorite keyword in the existing rel values wiki to make documents using it as `rel` value “conforming”.
And `image_src` has indeed been registered there, with the information that it is used to “specify a Webpage Icon for use by Facebook, Yahoo, Digg, etc.”, no specification has been identified but an article about it is linked to, and it is “probably redundant with rel=icon”.
Problem
Today I came across a `<link rel="image_src">` tag. I don't know about it, so I use google. Google tell me that this tag are similar to `og:image`. So I came to open graph main site to read about it http://ogp.me/, but i found nothing about link `rel="image_src"`. So this tag is replacement to meta `property="og:image"` or is in special tag in another specification ? How use this tag or for what is used?