Ya learn something new every day. Although I didn’t think so, and it plays hell with some HTML WYSIWYGs and link checkers, the following is a valid
src and also works with
<samp> <img src="//www.example.com/logo.gif" /> </samp>
When on a non-https site, it loads the image from
http://www.example.com/logo.gif. On an https secure site, it loads the image from
// means use the scheme part of the URI that is currently in use.