https://developer.mozilla.org/en-US/docs/Mozilla/Mobile/Viewport_meta_tag
http://www.w3schools.com/css/css_rwd_viewport.asp
The viewport is the user's visible area of a web page.
A typical mobile-optimized site contains something like the following:
<meta name="viewport" content="width=device-width, initial-scale=1">
The width property controls the size of the viewport. It can be set to a specific number of pixels like width=600 or to the special value device-width value which is the width of the screen in CSS pixels at a scale of 100%. (There are corresponding height and device-height values, which may be useful for pages with elements that change size or position based on the viewport height.)
The initial-scale property controls the zoom level when the page is first loaded. The maximum-scale, minimum-scale, and user-scalable properties control how users are allowed to zoom the page in or out.
tips
1.Do NOT use large fixed width elements
For example, if an image is displayed at a width wider than the viewport it can cause the viewport to scroll horizontally. Remember to adjust this content to fit within the width of the viewport.
在使用中发现一个bug：
viewport的width即使设置了固定值 ,但是设置了initial-scale这个属性后，会自动把width变成device-width(即使width不设置，也会默认设置width=device-width).
试验如下（chrome）中调试（页面内容按640宽度设置）：
1.width=640,initial-scale=1 不正常
2.单独 width=640 正常
3.单独 initial-scale=设备宽度／640 正常(写不写width=device-width都一样)
4.同时设置 width=640 和 initial-scale=设备宽度／640 正常
结论：按照设想，视口（viewport）宽度设置640 ，页面缩放是1，根据定义应该是正常的。但是试验后发现结果是另外一种情形。根据试验结果猜想应该是initial-scale影响了width的值(变为device-width)
