移动设备的屏幕比较小，它的viewport就是能用来渲染网页的可视矩形窗口（不包括地址栏以及底部操作栏）。
现代的移动设备浏览器基本上都能处理桌面网页，但是很显然移动屏幕那么小，要显示桌面网页，肯定需要经过一些特殊处理：
Certain classes of browser attempt to display desktop pages on a small screen
by automatically zooming the display. This can be problematic for applications
that have already been optimized for a small screen. The viewport meta tag
tells the device at what scale to render the page.
注：引自http://www.w3.org/TR/mwabp/#bp-viewport
一个典型的viewport meta tag设置如下：
1 | <meta name="viewport" content="width=device-width, initial-scale=1.0" /> |
把上面代码插到页面head标签内，告诉移动浏览器按照设备的100%宽度去渲染页面：
This setting informs the browser to always render the page at 100% (e.g. no
browser based scaling) and is appropriate for pages specifically designed for
the target screen-size.
更多viewport内容，请参考以下文章：
