固定网页宽度让手机自适应，这里不在iphone4下不能自适应
<meta name='viewport' content='width=640px, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0, user-scalable=no,target-densitydpi=device-dpi'>
<meta name="apple-mobile-web-app-capable" content="yes">
<meta name="apple-mobile-web-app-status-bar-style" content="black">
<meta name='format-detection' content='telephone=no'>
请注意：target-densitydpi=device-dpi
http://www.iinterest.net/2011/05/02/about-viewport/
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