[bes]

<input type="password">
<input style="display: none;">
<input type="button" value="show/hide">
<script>
window.onload = function () {
	function f(el1, el2) {
		el2.style.display  = 'inline';
		el1.style.display = 'none';
		el2.value = el1.value;
	}
	var els = document.body.children;
	els[2].onclick = function () {
		if (navigator.userAgent.indexOf('MSIE') != -1) {
			if (els[1].style.display != 'inline') {
				f(els[0], els[1]);
			} else {
				f(els[1], els[0])
			}
			return;
		}
		if (els[0].getAttribute('type') != 'text') {//
			els[0].setAttribute('type', 'text');
		} else {
			els[0].setAttribute('type', 'password');
		}
	}
}
</script>

либо через insertAdjacentHTML