The last FOUR versions of Internet Explorer all support querySelectorAll and the versions earlier than that are all effectively dead with almost no one using them any more - even IE9 is now effectively dead (although IE8 still has a few too many users to completely ignore - although many web sites are now moving toward letting those users see the pages the same as all the other long dead browser users see).
Of course using a polyfill means that you can still support users of prehistoric browsers where only those users have the script run extra slow because of the polyfill while everyone using modern browsers gets the full benefit of the modern coding in the script.
That would be awesome. Is it just a matter of including that file? Or does it need to be configured? I can’t seem to figure out if the whole thing should be added in the head or down underneath the markup where you’d normally put something that is DOM dependent.
They would need to be using IE7 or earlier for the code without the polyfill not to work.
Just include the script reference to the polyfill code with the rest of your scripts in the page. Polyfills are usually coded in such a way that they first test if the browser supports the command and only if it doesn’t do they then run their code to emulate the missing command.