ie8兼容canvas及svg

/////canvas
方案一
html5.js+excanvas.compiled.js+ie-css3.htc

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参考：
http://blog.csdn.net/huang100qi/article/details/7698647
https://lgtiamo.com/2016/05/29/use-the-html5-canvas-in-ie8/

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方案二
flashcanvas js

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http://www.uedsc.com/flashcanvas.html

方案三
ExplorerCanvas js

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http://lhdst-163-com.iteye.com/blog/1913876

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/////svg

方案一
Raphael js

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参考：
http://dmitrybaranovskiy.github.io/raphael/
http://www.cnblogs.com/lhb25/archive/2013/01/08/raphael-js-reference.html

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方案二
SVG和VML 兼容ie8-，svg+vml的js库vgraph.js
但是不支持ie11+

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参考：
http://www.cnblogs.com/hongru/archive/2011/06/18/2084215.html

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tips：所有向下兼容都会影响性能