href
without protocol (Protocol-relative URL) is used to remove the naggy warning of accessing unsecure content in IE.
But it'll cause double download in IE7 and IE8 (alas, WHY U NO SMART!!!).
The standard RFC3986 is mentioned in Paul Irish's article. But standard can only serve as a reference in web development, the browser vendors' implementations is more important. Websites like Can I Use, WebPlatform documents this info.
Do refer to these links:
http://www.paulirish.com/2010/the-protocol-relative-url/
http://blog.httpwatch.com/2010/02/10/using-protocol-relative-urls-to-switch-between-http-and-https/
http://www.stevesouders.com/blog/2010/02/10/5a-missing-schema-double-download/