Better insert before `document.documentElement.firstChild`

[mathiasbynens]

This kinda technique is used throughout has.js:

root = d.body || (function(){
    fake = true;
    return de.insertBefore(d.createElement("body"), de.firstChild);
}());

E.g. https://github.com/phiggins42/has.js/blob/master/detect/bugs.js#L102-105

Might be worthwhile to use de.firstElementChild || de.firstChild instead, in case the firstChild is a comment node and the browser doesn’t support inserting before comment nodes.

(Props to Paul Irish for pointing out the mistake in my <details>/<summary> feature test, and John-David Dalton for coming up with this elegant solution.)

[jdalton]

Imma try to apply this tonight :D