You can use scheme-less URLs (or protocol-relative URLs Show archive.org snapshot ) to have browsers use the current protocol (HTTP or HTTPS) when loading content referenced with such an URL.

A protocol relative URL doesn’t contain a protocol. For example, http://stevesouders.com/images/book-84x110.jpg becomes //stevesouders.com/images/book-84x110.jpg

Browsers substitute the protocol of the page itself for the resource’s missing protocol. Problem solved!

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But:

Internet Explorer 7 & 8 will download stylesheets twice if the http(s) protocol is missing.

So if you are working on a publicly accessible site, you can't use it for your stylesheet tags. Everything else works, though.