If your page gets called using https protocol, any relative path to an external css will be called using https protocol as well. Are you really need to encrypt/decrypt css contents? 😀 However, if you use absolute path referring to an external css, you can specify the protocol to use, generally http rather than https. Maybe I am wrong!
Please let me know whether or not my mental model is totally wrong.
Yes you are correct, relative paths will use the same protocol you’re currently on, fully qualified paths can include whatever protocol they want.
You typically want the relative behavior to act like it does, otherwise you’ll get a insecure-content warning if you’re loading
http://resources on ahttps://page in most browsers.