viewport 视口
<meta name="viewport" content="width=device-width,initial-scale=1.0,maximum-scale=1,minimum-scale=1,user-scalable=no"/>
width=device-width /*视口宽度等于屏幕宽度*/
initial-scale=1.0 /*页面初始缩放比例*/
maximum-scale=1 /*不允许放大窗口*/
minimum-scale=1 /*不允许缩小窗口*/
user-scalable=no /*禁止缩放*/
