event.srcElement is not working in firefox
참조:http://stackoverflow.com/questions/5301643/how-can-i-make-event-srcelement-work-in-firefox-and-what-does-it-mean

document.getElementById('login-username').onfocus = function(evt) {
   var element = ((window.event)?(event.srcElement):(evt.currentTarget));
   element.value = "";
}
---------------------

<input id="login-username" name="username" type="text"
					 value="username" />
<input id="login-password" name="password" type="password"
					value="password" />

<script type="text/javascript" charset="utf-8">
	var login_username = document.getElementById('login-username');
	login_username.onfocus = function(e) {
		var  elm = (window.event)? event.srcElement: e.currentTarget;
		if (elm.value === 'username') {
			elm.value = '';
		}
	}
	login_username.onblur = function(e) {
		var  elm = (window.event)? event.srcElement: e.currentTarget;
		if (elm.value === '') {
			elm.value = 'username';
		}
	}
	var login_password = document.getElementById('login-password');
	login_password.onfocus = function(e) {
		var  elm = (window.event)? event.srcElement: e.currentTarget;
		if (elm.value === 'password') {
			elm.value = '';
		}
	}
	login_password.onblur = function(e) {
		var  elm = (window.event)? event.srcElement: e.currentTarget;
		if (elm.value === '') {
			elm.value = 'password';
		}
	}
</script>