And it breaks unexpectedly (and sometimes hilariously) when people use it in html email, it turns into a file:// "link".
You can't have external css or js in an html email. [1] Are you talking about links? Or image tags?
[1] Well you can, but no one will fetch them.
The browser will load the resource from whatever protocol the current page is loaded from. So if the page is on https, the URL will be accessed as https, otherwise as http.
dagga, FYI, you are hellbanned. I am not sure why, your comments look reasonable. I would email a staff member or create a new account.
it will reuse the protocol of the current page, think http or https or spdy or whatever comes in the future.