Protocol relative URL / Scheme less URL advantages.

Nothing but all your URLs will be relative to the current page's protocol.
simple isn't it?

Just append URL(without protocol) with " // ", look at the example below.
Eg:   <img src="//"> 

Why do we need to use protocol relative URL / Scheme less URL 
If the browser is viewing current page through HTTPS, then it’ll request the resource(whose URL is scheme less) with the HTTPS protocol, otherwise it’ll request it with HTTP.
Yes same thing works for files which are viewed locally.
It’ll try to request the file with the file://protocol.