//example.com/img.png is a perfectly valid URI syntax as per RFC 3986: Section 4.2. It is relative to the current scheme, and … it can be very useful when switching between http and https, because you won’t need to explicitly specify the scheme.

~ Daniel Vassallo from, http://stackoverflow.com/questions/3583103/network-path-reference-uri-scheme-relative-urls

How is it possible I’ve never learned this?

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