On the page https://dev.twitter.com/docs/intents
first example code includes
It’s actually intentional. The first form is a “protocol-less” version of the URL, allowing the browser to selectively choose whether to use HTTP or HTTPS based on the context of the containing page. The primary downside to this form of notation is that they’re only valid when a HTML document is served from a web server (as opposed to a local file system, as is often the case in WYSIWYG HTML editing software).