css3提供了一些与当前viewpoint相关的元素，vw，vh，vim等。
“viewpoint” = window size vw = 1% of viewport width 1vh = 1% of viewport height 1vmin = 1vw or 1vh, 最小 1vmax = 1vw or 1vh, 最大
兼容性：chrome 20+/ safari 6+/ IE 10+ / FF 19+ / IOS 6+
DEMO地址：http://qianduannotes.sinaapp.com/test/fontResize.html （已经用JS修正重绘bug）
<!DOCTYPE HTML>
<html>
<head>
<meta charset="UTF-8">
<title>CSS3 Demo</title>
<style type="text/css">
#box { 10em; padding: 15px; font-size: 1em;}
#box2 { font-size: 4vw;}
</style>
</head>
<body>
<div id="box">
我是barret lee 我是barret lee 我是barret lee
</div>
<div id="box2">
我是barret lee 我是barret lee 我是barret lee
</div>
<script type="text/javascript">
window.onresize = function(){
var box2 = document.getElementById("box2");
box2.style["z-index"] = 1;
}
</script>
</body>
</html>