They are scheme relative urls.
https://stackoverflow.com/questions/...-relative-urls
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Scheme relative URLs (network-path references) are something that I've just found out about - where you don't specify the scheme of a URL and it picks it up from the current context.
For example: <img src="//domain.com/img.png" /> will resolve to https://domain.com/img.png if the current scheme is HTTPS or http://domain.com/img.png if it is not.
This seems like a very easy way to resolve those pesky problems of calling an external script or image on an SSL page without bringing up the dreaded error that some content on a page is not secure.
The benefit seems obvious, but what I don't seem to be able to find is a huge amount of information on this and was wondering if anyone had any experience or references about scheme relative URLs (good or bad)?
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This article references an article from 2010 telling people to use scheme ralative but was updated in 2016 to tell you why it is bad to use them:
https://jeremywagner.me/blog/stop-us...-relative-url/
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Paul Irish has since changed his stance, and now advises against using this URL scheme to reference cross-origin resources. His rationale is that while using the scheme on HTTPS pages is safe, the reverse is not true. Using the scheme on a non-secure page will retrieve cross-origin assets in a non-secure fashion. This isn't good, and it's rather easy to just slap an https:// protocol on your URLs to ensure that your site is completely secure.
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