viewport
代码大概是这样的：
<meta name="viewport" content="width=device-width, initial-scale=1">取值如下：
height=[pixel_value] | device-height
width=[pixel_value] | device-width
initial-scale=[float_value]
minimum-scale=[float_value]
maximum-scale=[float_value]
user-scalable=yes | no
target-densitydpi=[dpi_value] | device-dpi | high-dpi | medium-dpi | low-dpi
iOS专属META标记：
| name | content |
| apple-mobile-web-app-capable | yes | no |
| apple-mobile-web-app-status-bar-style | default | black | black-translucent |
| format-detection | telephone=no | yes |
| apple-mobile-web-app-title |