viewpoint
css3提供了一些与当前viewpoint相关的元素，vw，vh，vim等。
“viewpoint” = window sizevw = 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+
<!DOCTYPE HTML>
<html>
<head>
<meta charset="UTF-8">
<title>Demo</title>
<style type="text/css">
#text { font-size: 4vw;}
</style>
</head>
<body>
<div id="text">
测试文本测试文本
</div>
</body>
</html