Workaround for Mozilla bug #1276438 in Firefox

document-body.js

// Workaround for Mozilla bug #1276438 in Firefox
// See https://bugzilla.mozilla.org/show_bug.cgi?id=1276438
if (!Object.getOwnPropertyDescriptor(Document.prototype, "body"))
Object.defineProperty(Document.prototype, "body", {
	  enumerable: true
	, configurable: true
	, get() {
		return this.evaluate(
			  "/*[local-name()='html'][namespace-uri()='http://www.w3.org/1999/xhtml']"
			+ "/*[local-name()='body'][namespace-uri()='http://www.w3.org/1999/xhtml']"
			, this, null, XPathResult.FIRST_ORDERED_NODE_TYPE, null
		).singleNodeValue;
	}
	, set(replacement) {
		var body = this.body;
		body.parentElement.replaceChild(replacement, body);
	}
});