On several pages I saw in the last months following syntax for a address
<a href="//mynextpage.html">...</a>
What does this way of an url stay for? Is it a kind of wildcard for the protocol you have called this site?
It means, that the link reference uses the same protocol (http: or https:) like the current page. If you call the page with http: the link goes to http: as well, if you call the page with https: the link has https too.
This is most useful not only for links but for images, javascript and other external resources. It prevents problems with mixed (encrypted/unencrypted) content which provokes warnings in most browsers.