I still see this confusingly often, even with Twitter’s timeline embedding feature. Protocol relative URLs look like this:
<script async src="//platform.twitter.com/widgets.js" charset="utf-8"></script>
There is absolutely no reason to do that (anymore)! Take a look at the following table (perspective = the URL over which user the user opens the page) of the results:
|Scheme in URL||Browsed over: HTTP||Browsed over: HTTPS||Browsed over: Filesystem|
|http||=> http||=> http||=> http|
|https||=> https||=> https||=> https|
|//||=> http||=> https||(broken)|
As you can see, with protocol-relative the only “gain” we get (compared to linking via https) is that for non-secure users, they are offered the resource over an insecure channel. https used to be slow, and that was the only acceptable reason to use the protocol-relative URL - to gain some speed for folks who opened the URL over insecure channel.
https works everywhere (and is not slow anymore), so if the asset can be fetched over https it should be done so.
Using protocol-relative URLs you’re only downgrading the security of your users AND breaking any web pages that are opened from the filesystem. Shame on you!