Simple DOM diff :P

render.html

<script src="render.js"></script>
<script>
	var a = 0
	var b = 0
	var c = function () {
		return a + b
	}

	function render (e) {
		if (e.type === 'input') {
			if (e.target.id === 'a') {
				a = e.target.valueAsNumber
				a = isNaN(a) ? 0 : a
			}
			if (e.target.id === 'b') {
				b = e.target.valueAsNumber
				b = isNaN(b) ? 0 : b
			}
		}
		return '<input id="a" type="number">'
			+ ' + <input id="b" type="number">'
			+ ' = <input value="' + c() + '" readonly>'
	}
</script>

render.js

function render () {
	return ''
}

addEventListener('DOMContentLoaded', function (e) {
	var slice = [].slice
	var timeout

	function updateNode (previous, next) {
		var i, len, attr, p, pl, n, nl
		if (previous.nodeType !== next.nodeType
				|| previous.nodeName !== next.nodeName) {
			previous.parentNode.replaceChild(next, previous)
		} else if (previous.nodeType === Node.TEXT_NODE) {
			if (previous.textContent !== next.textContent) {
				previous.textContent = next.textContent
			}
		} else if (previous.nodeType === Node.ELEMENT_NODE) {
			for (i = next.attributes.length - 1; i > -1; i--) {
				attr = next.attributes[i]
				if (previous.getAttribute(attr.name) !== attr.value) {
					previous.setAttribute(attr.name, attr.value)
				}
			}
			for (i = previous.attributes.length - 1; i > -1; i--) {
				attr = previous.attributes[i]
				if (!next.hasAttribute(attr.name)) {
					previous.removeAttribute(attr.name)
				}
			}
			p  = slice.call(previous.childNodes)
			n  = slice.call(next.childNodes)
			pl = p.length
			nl = n.length
			if (pl < nl) {
				for (i = pl; i < nl; i++) {
					previous.appendChild(n[i])
				}
			} else if (pl > nl) {
				for (i = nl; i < pl; i++) {
					previous.removeChild(p[i])
				}
			}
			len = Math.min(pl, nl)
			for (i = 0; i < len; i++) {
				updateNode(p[i], n[i])
			}
		}
	}

	function updateBody (event) {
		clearTimeout(timeout)
		timeout = setTimeout(function () {
			var next = document.createElement('body')
			next.innerHTML = render(event)
			updateNode(document.body, next)
		}, 1000 / 60)
	}

	addEventListener('keydown', updateBody)
	addEventListener('keyup',   updateBody)
	addEventListener('input',   updateBody)
	addEventListener('submit',  updateBody)

	updateBody(e)
})