Quick demo of how a JavaScript based selector engine might work.

select.js

function select(selector) {
	var elements = [document],
		queryStart = 0,
		callQueue = [],
		temp,
		i;

	for (i = 0; i < selector.length; i++) {
		switch (selector[i]) {
			case '#':
				queryStart = i + 1;
				method = 'getElementById';
				break;
			case '.':
				queryStart = i + 1;
				method = 'getElementsByClassName';
				break;
			case ' ':
				callQueue.push({
					method: method,
					argument: selector.substring(queryStart, i)
				});
				queryStart = i + 1;
				break;
		}
	}

	callQueue.push({
		method: method,
		argument: selector.substring(queryStart, i)
	});

	while (callQueue.length) {
		temp = callQueue.shift();
		for (i = elements.length - 1; i >= 0; i--) {
			temp = elements.shift()[temp.method](temp.argument);
			elements.push.apply(elements, temp instanceof HTMLCollection ? temp : [temp]);
		}
	}

	return elements;
}