_usage.js

var preact = require('preact');
var createRenderer = require('./preact-dom-renderer');

// set up a rendering target
var renderer = createRenderer();

// render some stuff into the internal DOM
renderer.render(preact.h(PreactApp, pageConfig));

// get a snapshot of the current HTML
console.log( renderer.html() );

preact-dom-renderer.js

var preact = require('preact');
var undom = require('undom');

/**
 *	Prototype stateful server renderer.
 */
var doc;
module.exports = function createRenderer() {
	if (!doc) {
		doc = undom();
		Object.assign(global, doc.defaultView);
	}

	var root, parent = doc.createElement('x-root');
	doc.body.appendChild(parent);

	return {
		render: function(jsx) {
			root = preact.render(jsx, parent, root);
			return this;
		},
		html: function() {
			return serializeHtml(root);
		}
	};
}

function serializeHtml(el) {
	if (el.nodeType===3) return esc(el.nodeValue);
	var name = String(el.nodeName).toLowerCase(),
		str = '<'+name,
		hasClass = false,
		c, i;
	for (i=0; i<el.attributes.length; i++) {
        let name = el.attributes[i].name;
		if (name==='class') hasClass = true;
	  str += ' '+name+'="'+esc(el.attributes[i].value)+'"';
	}
	if (el.className && !hasClass) str += ' class="'+el.className+'"';
	str += '>';
	for (i=0; i<el.childNodes.length; i++) {
		c = serializeHtml(el.childNodes[i]);
		if (c) str += c;
	}
	return str + '</'+name+'>';
}

function esc(str) { return String(str).replace(/[&<>"']/g, escMap); }
function escMap(s) { return '&'+map[s]+';'; }
var map = {'&':'amp','<':'lt','>':'gt','"':'quot',"'":'apos'};

Fixed!