favicon支持的图片格式
为网站设置favicon有两种方式：
1、网站根目录下放置名为favicon.ico的图片，浏览器就会自动获取；
2、在页面中通过<link rel="shortcut icon" href="http://example.com/myicon.ico" />.
对于favicon来说，一般都是使用ico格式，不过对于根目录的favicon图片chrome、firefox、IE11等现代浏览器都支持PNG、GIF（无动画）格式。对于通过link来设置的favicon格式，IE7+、现代浏览器都支持PNG、GIF格式，不过在IE7下图片效果会很差。
对于苹果设备，我们可以通过在head里写入<link rel="apple-touch-icon" href="somepath/image.png" />、<link rel="apple-touch-icon-precomposed" href="somepath/image.png" />来设置。
| Google Chrome | Internet Explorer | Firefox | Opera | Safari | |
|---|---|---|---|---|---|
<link rel="shortcut icon" href="http://example.com/myicon.ico" /> |
Yes[4] | Yes[4][19] | Yes[4] | Yes[4] | Yes |
<link rel="icon" href="http://example.com/image.ico" /> |
Yes | Yes (from IE 11)[20] | Yes | Yes | Yes |
<link rel="icon" type="image/vnd.microsoft.icon" href="http://example.com/image.ico" /> |
Yes | Yes (from IE 9)[20] | Yes | Yes | Yes |
<link rel="icon" type="image/png" href="http://example.com/image.png" /> |
Yes | Yes (from IE 11)[20] | Yes | Yes | Yes |
<link rel="icon" type="image/gif" href="http://example.com/image.gif" /> |
Yes | Yes (from IE 11)[20] | Yes | Yes | Yes |
<link rel="icon" type="image/x-icon" href="http://example.com/image.ico"/> |
Yes[4] | Yes (from IE 9)[26] | Yes[4] | Yes[4] | Yes |
favicon.ico located in the website's root |
Yes | Yes | Optional[27] | Optional[28] | Yes |
| precedence: prefer root or (X)HTML linked version | linked[22] | linked[22] | linked[22] | ? | ? |
参考文档：
http://en.wikipedia.org/wiki/Favicon
http://msdn.microsoft.com/en-us/library/ms537656(v=vs.85).aspx
http://stackoverflow.com/questions/1344122/favicon-png-vs-favicon-ico-why-should-i-use-pngs-instead-of-icos
http://www.webmasterworld.com/html/3701134.htm
http://blogs.msdn.com/b/ieinternals/archive/2013/09/08/internet-explorer-favicons-png-link-rel-icon-caching.aspx
http://blogs.msdn.com/b/jeffdav/archive/2007/03/01/why-doesn-t-the-favicon-for-my-site-appear-in-ie7.aspx
