使用iepngfix.htc文件解决png图片在IE6下不透明的问题。
什么是htc文件？
从5.5版本开始，Internet Explorer（IE）开始支持Web 行为的概念。这些行为是由后缀名为.htc的脚本文件描述的，它们定义了一套方法和属性，程序员几乎可以把这些方法和属性应用到HTML页面上的任何元素上去。Web 行为是非常伟大的因为它们允许程序员把自定义的功能“连接”到现有的元素和控件，而不是必须让用户下载二进制文件（例如ActiveX 控件）来完成这个功能。Web 行为还是推荐的扩展IE对象模型和控件集的方法。 更多介绍
下面这个网站给出了解决方法：
http://www.twinhelix.com/css/iepngfix/
http://www.twinhelix.com/css/iepngfix/demo/
一点介绍：
This is a IE5.5+ “behavior” that automatically adds near-native PNG support to MSIE 5.5 and 6.0 without any changes to the HTML document itself. Supported features include:
- Automatic activation of transparency for PNGs in the page.
- Support for <IMG SRC=””> elements.
- Support for background PNG images (unlike many other scripts!)
- Support for CSS1 background repeat and position (via optional add-on)
- Background images can be defined inline or in external stylesheets.
- Automatically handles changed SRC/background via normal JavaScript (e.g. mouseover rollovers) – no special coding needed.
- Change support includes CSS ‘className’ changes on elements.
- Incorporates automatic workaround for <a href=””> elements within PNG-background elements.
- Tiny script (for fast downloads).
- Licensed under a Free Software license.