I wonder about the use of a non-standard HTML tag, <github>. It works for styling purpose, and personally I don't have an issue with it - but it may put off some people due to it being "invalid" HTML..?
I can see the novelty of it ("Look at my fancy <github> tag!"), but it would definitely be more helpful if it used a standard HTML element (preferably <div>). I've personally opted for this implementation on my own projects: https://github.com/tholman/github-corners, which are just SVGs which makes it super portable.
I don't think even a div is necessary - yours is pretty succinct: an anchor and an image. It could potentially even just be an anchor with text and let css hide the text, apply the image etc.
