这篇文章介绍css method to achieve background transparency and text opacity
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)
css3的opacity
兼容性：IE6、7、8不支持，IE9及以上版本和标准浏览器都支持
使用说明：设置opacity元素的所有后代元素会随着一起具有透明性，一般用于调整图片或者模块的整体不透明度
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>opacity</title>
<style>
*{
padding: 0;
margin: 0;
}
body{
padding: 50px;
background: url(img/bg.png) 0 0 repeat;
}
.demo{
padding: 25px;
background-color:#000000;
opacity: 0.2;
}
.demo p{
color: #FFFFFF;
}
</style>
</head>
<body>
<p class="demo">
<p>背景透明，文字也透明</p>
</p>
</html>
那么使用opacity实现《背景透明，文字不透明》是不可取的。
css3的rgba
兼容性：IE6、7、8不支持，IE9及以上版本和标准浏览器都支持
使用说明：设置颜色的不透明度，一般用于调整background-color、color、box-shadow等的不透明度。
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>css3的rgba</title>
<style>
*{
padding: 0;
margin: 0;
}
body{
padding: 50px;
background: url(img/bg.png) 0 0 repeat;
}
.demo{
padding: 25px;
background-color:#000000;/* IE6和部分IE7内核的浏览器(如QQ浏览器)下颜色被覆盖 */
background-color:rgba(0,0,0,0.2); /* IE6和部分IE7内核的浏览器(如QQ浏览器)会读懂，但解析为透明 */
}
.demo p{
color: #FFFFFF;
}
</style>
</head>
<body>
<p class="demo">
<p>背景透明，文字也透明</p>
</p>
</html>在background-color中使用rgba，标准浏览器中，背景透明，文字不透明，展现如下：
很奇葩的是，IE6和部分IE7内核的浏览器(如QQ浏览器)会读懂rgba，解析后颜色为透明，其实应该是null
那么使用opacity实现背景透明，文字不透明是可取的。
IE专属滤镜 filter:Alpha(opacity=x)
使用说明：IE浏览器专属，问题多多，本文以设置背景透明为例子，如下：
仅支持IE6、7、8、9，在IE10版本被废除
在IE6、7中，需要激活IE的haslayout属性(如：*zoom:1或者*overflow:hidden)，让它读懂filter:Alpha
在IE6、7、8中，设置了filter:Alpha的元素，父元素设置position:static(默认属性)，其子元素为相对定位，可让子元素不透明
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>opacity</title>
<style>
*{
padding: 0;
margin: 0;
}
body{
padding: 50px;
background: url(img/bg.png) 0 0 repeat;
}
.demo{
padding: 25px;
background: #000000;
filter:Alpha(opacity=50);/* 只支持IE6、7、8、9 */
position:static; /* IE6、7、8只能设置position:static(默认属性) ，否则会导致子元素继承Alpha值 */
*zoom:1; /* 激活IE6、7的haslayout属性，让它读懂Alpha */
}
.demo p{
color: #FFFFFF;
position: relative;/* 设置子元素为相对定位，可让子元素不继承Alpha值，保证字体颜色不透明 */
}
</style>
</head>
<body>
<p class="demo">
<p>背景透明，文字不透明</p>
</p>全兼容的方案
上以上3点分析可知，设置透明背景文字不透明，可采用的属性有rgba和IE的专属滤镜filter:Alpha，其兼容性如下图所示：
针对IE6、7、8浏览器，我们可以采用fiter:Alpha，针对标准浏览器我们采用rgba，那么问题来了，IE9浏览器2个属性都支持，一起使用会重复降低不透明度...
那么，如何只对IE6、7、8使用fiter:Alpha如何实现呢？2年前写过《CSS hack整理》一文，最新我也做了点更新，里面有IE的相关hack，找到只支持IE 6、7、8的方案，如下：
/* 只支持IE6、7、8 */
@media \0screen\,screen\9 {...}
ok，所有问题都解决了，全部代码如下：
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>背景透明，文字不透明</title>
<style>
*{
padding: 0;
margin: 0;
}
body{
padding: 50px;
background: url(img/bg.png) 0 0 repeat;
}
.demo{
padding: 25px;
background-color: rgba(0,0,0,0.5);/* IE9、标准浏览器、IE6和部分IE7内核的浏览器(如QQ浏览器)会读懂 */
}
.demo p{
color: #FFFFFF;
}
@media \0screen\,screen\9 {/* 只支持IE6、7、8 */
.demo{
background-color:#000000;
filter:Alpha(opacity=50);
position:static; /* IE6、7、8只能设置position:static(默认属性) ，否则会导致子元素继承Alpha值 */
*zoom:1; /* 激活IE6、7的haslayout属性，让它读懂Alpha */
}
.demo p{
position: relative;/* 设置子元素为相对定位，可让子元素不继承Alpha值 */
}
}
</style>
</head>
<body>
<p class="demo">
<p>背景透明，文字不透明</p>
</p>
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