https://blogs.msdn.microsoft.com/dotnet/2014/12/04/introduci...
Don't think of .NET Standard as another .NET platform. Think of it as the standard that ties them all together. It's similar in spirit to POSIX. It's not another Unix, it's a way to write code that works on multiple Unixes.
As an aside, the content type for that URL is application/octet-stream, so Firefox won’t view the image but insists on downloading it. It needs to be image/png to be viewable in browser I believe.
The point being that ANSI C didn't took the rest of UNIX APIs to other OSes as part of the specification, but in the end most OSes that adopted C ended up having to support POSIX as well as a means to have some kind of usable C library common across OSes.