Level 6: You can exclude the protocol entirely (eg: "//news.ycombinator.com")

This will ensure the browser uses the "current protocol" as in if your website is browseable from http all request //www...com will be http and if your page is fetched using https, all resources starting with //www.hn.com will be loaded using https

if your website was reachable from protocol xyz://mydomain.com, all resources starting with // would be fetched using the xyz:// protocol