viewport
<meta name="viewport" content="key=value, key=value">
有6个关键值
width
height
initial-scale
minimum-scale
maximum-scale
user-scalable
width
<meta name="viewport" content="width=320">
viewport为320个CSS像素；
initial-scale
<meta name="viewport" content="width=320, initial-scale=1">
当窗口小于320px时，viewport为320个CSS像素；当窗口大于320px时，viewport的宽度等于设备像素（1:1）；
参考
http://dev.w3.org/csswg/css-device-adapt/#atviewport-rule
https://developer.mozilla.org/zh-CN/docs/Mobile/Viewport_meta_tag
http://segmentfault.com/a/1190000002685485