cssで画面中央へ常に表示する

LightBoxっぽい感じで。prototype.js,jQuery使ってません。IE6無理　そのたの主要ブラウザ(chrome,Fx,IE7+8,Opera,Safari,Lunascape)はOK

div#certification {
	position: fixed!important;
	position: absolute;
	top:0;
	left:0;
	width:100%;
	height:100%;
	z-index:100;
	text-align: center;
	line-height: 0;
}

div#glayer {
	position: absolute;
	top:0;
	left:0;
	margin: 0;
	padding: 50%;
	background-color:#1F1F1F;
	-moz-opacity: 0.8;
	opacity:.80;
	filter: alpha(opacity=80);
}

div#certification_window_center {
	position: absolute;
	top:50%;
	left:50%;
}

div#certification_window {
	position: relative;
	padding: 20px;
	left: -200px;
	top: -175px;
	width: 360px;
	height: 310px;
	text-align:center;
	background-image: url('./certification_window.png');
}

var certification = {
	popup:function certification(){
		var certification = document.createElement('div');
		certification.id = "certification";

		var glayer = document.createElement('div');
		glayer.id = "glayer";
		certification.appendChild(glayer);

		var certification_window_center = document.createElement('div');
		certification_window_center.id = "certification_window_center";
		certification.appendChild(certification_window_center);
		
		var certification_window = document.createElement('div');
		certification_window.id = "certification_window";
		certification_window.innerHTML = 'ssssssssssss'
					+ '<input type="button" value="yes" onclick="certification.upload()">'
					+ '<input type="button" value="no" onclick="certification.cancel()">';
		certification_window_center.appendChild(certification_window);
		
		var body = document.getElementsByTagName('body');
		body.item(0).appendChild(certification);
		return false;
	},
	cancel:function upload_cancel() {
		var popup = document.getElementById('certification');
		if (popup != null) {
			var parent = document.getElementById('certification').parentNode;
			parent.removeChild(popup);
		}
	},
	upload:function upload_movie() {
		var certification_window = document.getElementById('certification_window');
		if (certification_window == null) {
			return false;
		}
		certification_window.innerHTML = 'sssssss';
	}
}