Urls with two leading slashes and without schema

Sometimes I see script src or image src without schema, only two leading slashes //

From http://www.performable.com/ home page

<script src="//d2f7h8c8hc0u7y.cloudfront.net/performable/pax/8uEvsz.js"><开发者_运维技巧/script>

http://www.hellobar.com/ home page - and their integration code

<script type="text/javascript" src="//www.hellobar.com/hellobar.js"></script>

I've integrated both on my site and see errors in apache log similar to

www.mysite.com//www.hellobar.com/hellobar.js not found

However they're rare - less than 1%.

I think double leading slashes is a replacement for http\https and such links will use current page schema. Can someone confirm that?

Also, which browsers have support for that urls and which have problems?

This was discussed multiple times here. Yes it is safe and yes all browsers support this.