实现透明的css方法通常有以下3种方式（以下是不透明度都为80%的写法）
css3的opacity:x，x 的取值从 0 到 1，如opacity: 0.8
css3的rgba(red, green, blue, alpha)，alpha的取值从 0 到 1，如rgba(255,255,255,0.8)
IE专属滤镜 filter:Alpha(opacity=x)，x 的取值从 0 到 100，如filter:Alpha(opacity=80)
单独不能实现，头部加段JS代码就可以使PNG透明了<script language="javascript">
function correctPNG()
{
for(var i=0i<document.images.lengthi++)
{
var img = document.images[i]
var imgName = img.src.toUpperCase()
if (imgName.substring(imgName.length-3, imgName.length) == "PNG")
{
var imgID = (img.id) ? "id='" + img.id + "' " : ""
var imgClass = (img.className) ? "class='" + img.className + "' " : ""
var imgTitle = (img.title) ? "title='" + img.title + "' " : "title='" + img.alt + "' "
var imgStyle = "display:inline-block" + img.style.cssText
if (img.align == "left") imgStyle = "float:left" + imgStyle
if (img.align == "right") imgStyle = "float:right" + imgStyle
if (img.parentElement.href) imgStyle = "cursor:hand" + imgStyle
var strNewHTML = "<span " + imgID + imgClass + imgTitle
+ " style=\"" + "width:" + img.width + "pxheight:" + img.height + "px" + imgStyle + ""
+ "filter:progid:DXImageTransform.Microsoft.AlphaImageLoader"
+ "(src=\'" + img.src + "\', sizingMethod='scale')\"></span>"
img.outerHTML = strNewHTML
i = i-1
}
}
}
window.attachEvent("onload", correctPNG)
</script>
