依据浏览器（ie）不同版本执行引用js

<!--[if IE 6]>
<script src="DD_belatedPNG.js"></script>
<script>
/* EXAMPLE */
DD_belatedPNG.fix('.png_bg');

/* string argument can be any CSS selector */
/* .png_bg example is unnecessary */
/* change it to what suits you! */
</script>
<![endif]-->

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参考 http://msdn.microsoft.com/en-us/library/ms537512.aspx