You may already know this, in which case you ran across it randomly, or are the kind of developer that reads specs. But if you haven’t run across this, prepare for a face-palm moment wondering why you didn’t know this years ago.
Using a uri like the following tells the browser to get the content over whatever protocol the current document was fetched from.
Think of all the little annoying server side script you’ve written to determine the protocol based on the request variables in order to avoid security warnings. Never again. Enjoy.
One warning… don’t do this in phonegap.
More detail, and links to rfcs at stackoverflow.