转自：
http://www.mangguo.org/magic-css-element-opacity/
1、HTML 元素透明
其实本身，CSS 实现元素透明是件容易事儿。直接上代码：
opacity:.5
opacity 指的是不透明度，取值为 0~1 之间，1 表示完全不透明，0 表示完全透明。
A 级浏览器基本都支持 opacity 属性，但碰上 IE，总没好事。不过 IE 有滤镜，可以帮助我们拐弯抹角地搞定不透明度：
filter:alpha(opacity=50);
注：事实上会碰到透明背景层，而 opacity 属性是会继承的，避免这个问题，需要结合定位来实现。
在 CSS3 中，还可以使用 HSLA（色调、饱和、亮度、透明度）或者 RGBA（红、绿、蓝、透明度）来实现元素透明。例如：
background: hsla(0,100%,50%,0.5);
background: rgba(0,0,0,0.5);
2、背景图片透明
有一个值得关注的问题是，PNG8 格式的图片在 IE6 下仅支持全透明，也就是说，IE6 下使用PNG8 无法实现背景的半透明效果。透不透明其实有时候无所谓，但问题是全透明状态下，会导致图片出现恶心的毛边。虽然可以在导出PNG8 时，通过选择杂边颜色或者去除杂边的方式解决，但并不完美。
可喜的是，PNG24 支持全透明，所以PNG24 就很威猛了，在图片透明的方案中，它绝对是武力指数最高的。
但碰上 IE，另一个恶心的问题又出现了，IE6 不支持支持半透明的PNG24 透明图片（可能有点拗口）。
咋办？滤镜！
直接上代码：
_background:none;_filter:progid:DXImageTransform.Microsoft.AlphaImageLoader(src=\'mangguo.png\',sizingMethod=\'crop\');
但碰上 IE6，又悲剧了。在 CSS 文档和页面地址不一致时（比如很多网站会将静态资源放置于和站点本身不同域的 CDN 服务器上），会导致滤镜引用的图片不可获取。所以一旦资源跨域，必须保证图片地址为绝对路径。
看来用png24 加滤镜就能解决大多图片透明问题了；例如：
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
<style type="text/css">
.box1,.box2,.box3,.box4 { width:200px; height:200px;}
.box2 { position:absolute; top:0; left:0; background:url(percent5.png) no-repeat}
.box4 { position:absolute; top:200px; left:0; background: url(24.png) no-repeat;
_background:none;
_filter:progid:DXImageTransform.Microsoft.AlphaImageLoader(src=\'24.png\',sizingMethod=\'crop\')}
</style>
</head>
<body>
<div class="box1">你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！</div>
<div class="box2"></div>
<div class="box3">你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！你好！</div>
<div class="box4"></div>
</body>
</html>
3.今天又碰到一个 半透明的图像，需要切换背景图片的位置，这样就涉及到一个js 脚本：dd_png.js(可以百度一下 ，很多)
样式和之前写的没有两样，在文档中加入：
<!--[if IE 6]>
<script src="dd_png.js"></script>
<script>
DD_belatedPNG.fix(\'.png_bg\'); //应用背景的类名
</script>
<![endif]-->
完美实现！
附：dd_png.js源码：
/**
* DD_belatedPNG: Adds IE6 support: PNG images for CSS background-image and HTML <IMG/>.
* Author: Drew Diller
* Email: drew.diller@gmail.com
* URL: http://www.dillerdesign.com/experiment/DD_belatedPNG/
* Version: 0.0.8a
* Licensed under the MIT License: http://dillerdesign.com/experiment/DD_belatedPNG/#license
*
* Example usage:
* DD_belatedPNG.fix(\'.png_bg\'); // argument is a CSS selector
* DD_belatedPNG.fixPng( someNode ); // argument is an HTMLDomElement
**/
/*
PLEASE READ:
Absolutely everything in this script is SILLY. I know this. IE\'s rendering of certain pixels doesn\'t make sense, so neither does this code!
*/
var DD_belatedPNG = {
ns: \'DD_belatedPNG\',
imgSize: {},
delay: 10,
nodesFixed: 0,
createVmlNameSpace: function () { /* enable VML */
if (document.namespaces && !document.namespaces[this.ns]) {
document.namespaces.add(this.ns, \'urn:schemas-microsoft-com:vml\');
}
},
createVmlStyleSheet: function () { /* style VML, enable behaviors */
/*
Just in case lots of other developers have added
lots of other stylesheets using document.createStyleSheet
and hit the 31-limit mark, let\'s not use that method!
further reading: http://msdn.microsoft.com/en-us/library/ms531194(VS.85).aspx
*/
var screenStyleSheet, printStyleSheet;
screenStyleSheet = document.createElement(\'style\');
screenStyleSheet.setAttribute(\'media\', \'screen\');
document.documentElement.firstChild.insertBefore(screenStyleSheet, document.documentElement.firstChild.firstChild);
if (screenStyleSheet.styleSheet) {
screenStyleSheet = screenStyleSheet.styleSheet;
screenStyleSheet.addRule(this.ns + \'\\:*\', \'{behavior:url(#default#VML)}\');
screenStyleSheet.addRule(this.ns + \'\\:shape\', \'position:absolute;\');
screenStyleSheet.addRule(\'img.\' + this.ns + \'_sizeFinder\', \'behavior:none; border:none; position:absolute; z-index:-1; top:-10000px; visibility:hidden;\'); /* large negative top value for avoiding vertical scrollbars for large images, suggested by James O\'Brien, http://www.thanatopsic.org/hendrik/ */
this.screenStyleSheet = screenStyleSheet;
/* Add a print-media stylesheet, for preventing VML artifacts from showing up in print (including preview). */
/* Thanks to R閙i Pr関ost for automating this! */
printStyleSheet = document.createElement(\'style\');
printStyleSheet.setAttribute(\'media\', \'print\');
document.documentElement.firstChild.insertBefore(printStyleSheet, document.documentElement.firstChild.firstChild);
printStyleSheet = printStyleSheet.styleSheet;
printStyleSheet.addRule(this.ns + \'\\:*\', \'{display: none !important;}\');
printStyleSheet.addRule(\'img.\' + this.ns + \'_sizeFinder\', \'{display: none !important;}\');
}
},
readPropertyChange: function () {
var el, display, v;
el = event.srcElement;
if (!el.vmlInitiated) {
return;
}
if (event.propertyName.search(\'background\') != -1 || event.propertyName.search(\'border\') != -1) {
DD_belatedPNG.applyVML(el);
}
if (event.propertyName == \'style.display\') {
display = (el.currentStyle.display == \'none\') ? \'none\' : \'block\';
for (v in el.vml) {
if (el.vml.hasOwnProperty(v)) {
el.vml[v].shape.style.display = display;
}
}
}
if (event.propertyName.search(\'filter\') != -1) {
DD_belatedPNG.vmlOpacity(el);
}
},
vmlOpacity: function (el) {
if (el.currentStyle.filter.search(\'lpha\') != -1) {
var trans = el.currentStyle.filter;
trans = parseInt(trans.substring(trans.lastIndexOf(\'=\')+1, trans.lastIndexOf(\')\')), 10)/100;
el.vml.color.shape.style.filter = el.currentStyle.filter; /* complete guesswork */
el.vml.image.fill.opacity = trans; /* complete guesswork */
}
},
handlePseudoHover: function (el) {
setTimeout(function () { /* wouldn\'t work as intended without setTimeout */
DD_belatedPNG.applyVML(el);
}, 1);
},
/**
* This is the method to use in a document.
* @param {String} selector - REQUIRED - a CSS selector, such as \'#doc .container\'
**/
fix: function (selector) {
if (this.screenStyleSheet) {
var selectors, i;
selectors = selector.split(\',\'); /* multiple selectors supported, no need for multiple calls to this anymore */
for (i=0; i<selectors.length; i++) {
this.screenStyleSheet.addRule(selectors[i], \'behavior:expression(DD_belatedPNG.fixPng(this))\'); /* seems to execute the function without adding it to the stylesheet - interesting... */
}
}
},
applyVML: function (el) {
el.runtimeStyle.cssText = \'\';
this.vmlFill(el);
this.vmlOffsets(el);
this.vmlOpacity(el);
if (el.isImg) {
this.copyImageBorders(el);
}
},
attachHandlers: function (el) {
var self, handlers, handler, moreForAs, a, h;
self = this;
handlers = {resize: \'vmlOffsets\', move: \'vmlOffsets\'};
if (el.nodeName == \'A\') {
moreForAs = {mouseleave: \'handlePseudoHover\', mouseenter: \'handlePseudoHover\', focus: \'handlePseudoHover\', blur: \'handlePseudoHover\'};
for (a in moreForAs) {
if (moreForAs.hasOwnProperty(a)) {
handlers[a] = moreForAs[a];
}
}
}
for (h in handlers) {
if (handlers.hasOwnProperty(h)) {
handler = function () {
self[handlers[h]](el);
};
el.attachEvent(\'on\' + h, handler);
}
}
el.attachEvent(\'onpropertychange\', this.readPropertyChange);
},
giveLayout: function (el) {
el.style.zoom = 1;
if (el.currentStyle.position == \'static\') {
el.style.position = \'relative\';
}
},
copyImageBorders: function (el) {
var styles, s;
styles = {\'borderStyle\':true, \'borderWidth\':true, \'borderColor\':true};
for (s in styles) {
if (styles.hasOwnProperty(s)) {
el.vml.color.shape.style[s] = el.currentStyle[s];
}
}
},
vmlFill: function (el) {
if (!el.currentStyle) {
return;
} else {
var elStyle, noImg, lib, v, img, imgLoaded;
elStyle = el.currentStyle;
}
for (v in el.vml) {
if (el.vml.hasOwnProperty(v)) {
el.vml[v].shape.style.zIndex = elStyle.zIndex;
}
}
el.runtimeStyle.backgroundColor = \'\';
el.runtimeStyle.backgroundImage = \'\';
noImg = true;
if (elStyle.backgroundImage != \'none\' || el.isImg) {
if (!el.isImg) {
el.vmlBg = elStyle.backgroundImage;
el.vmlBg = el.vmlBg.substr(5, el.vmlBg.lastIndexOf(\'")\')-5);
}
else {
el.vmlBg = el.src;
}
lib = this;
if (!lib.imgSize[el.vmlBg]) { /* determine size of loaded image */
img = document.createElement(\'img\');
lib.imgSize[el.vmlBg] = img;
img.className = lib.ns + \'_sizeFinder\';
img.runtimeStyle.cssText = \'behavior:none; position:absolute; left:-10000px; top:-10000px; border:none; margin:0; padding:0;\'; /* make sure to set behavior to none to prevent accidental matching of the helper elements! */
imgLoaded = function () {
this.width = this.offsetWidth; /* weird cache-busting requirement! */
this.height = this.offsetHeight;
lib.vmlOffsets(el);
};
img.attachEvent(\'onload\', imgLoaded);
img.src = el.vmlBg;
img.removeAttribute(\'width\');
img.removeAttribute(\'height\');
document.body.insertBefore(img, document.body.firstChild);
}
el.vml.image.fill.src = el.vmlBg;
noImg = false;
}
el.vml.image.fill.on = !noImg;
el.vml.image.fill.color = \'none\';
el.vml.color.shape.style.backgroundColor = elStyle.backgroundColor;
el.runtimeStyle.backgroundImage = \'none\';
el.runtimeStyle.backgroundColor = \'transparent\';
},
/* IE can\'t figure out what do when the offsetLeft and the clientLeft add up to 1, and the VML ends up getting fuzzy... so we have to push/enlarge things by 1 pixel and then clip off the excess */
vmlOffsets: function (el) {
var thisStyle, size, fudge, makeVisible, bg, bgR, dC, altC, b, c, v;
thisStyle = el.currentStyle;
size = {\'W\':el.clientWidth+1, \'H\':el.clientHeight+1, \'w\':this.imgSize[el.vmlBg].width, \'h\':this.imgSize[el.vmlBg].height, \'L\':el.offsetLeft, \'T\':el.offsetTop, \'bLW\':el.clientLeft, \'bTW\':el.clientTop};
fudge = (size.L + size.bLW == 1) ? 1 : 0;
/* vml shape, left, top, width, height, origin */
makeVisible = function (vml, l, t, w, h, o) {
vml.coordsize = w+\',\'+h;
vml.coordorigin = o+\',\'+o;
vml.path = \'m0,0l\'+w+\',0l\'+w+\',\'+h+\'l0,\'+h+\' xe\';
vml.style.width = w + \'px\';
vml.style.height = h + \'px\';
vml.style.left = l + \'px\';
vml.style.top = t + \'px\';
};
makeVisible(el.vml.color.shape, (size.L + (el.isImg ? 0 : size.bLW)), (size.T + (el.isImg ? 0 : size.bTW)), (size.W-1), (size.H-1), 0);
makeVisible(el.vml.image.shape, (size.L + size.bLW), (size.T + size.bTW), (size.W), (size.H), 1 );
bg = {\'X\':0, \'Y\':0};
if (el.isImg) {
bg.X = parseInt(thisStyle.paddingLeft, 10) + 1;
bg.Y = parseInt(thisStyle.paddingTop, 10) + 1;
}
else {
for (b in bg) {
if (bg.hasOwnProperty(b)) {
this.figurePercentage(bg, size, b, thisStyle[\'backgroundPosition\'+b]);
}
}
}
el.vml.image.fill.position = (bg.X/size.W) + \',\' + (bg.Y/size.H);
bgR = thisStyle.backgroundRepeat;
dC = {\'T\':1, \'R\':size.W+fudge, \'B\':size.H, \'L\':1+fudge}; /* these are defaults for repeat of any kind */
altC = { \'X\': {\'b1\': \'L\', \'b2\': \'R\', \'d\': \'W\'}, \'Y\': {\'b1\': \'T\', \'b2\': \'B\', \'d\': \'H\'} };
if (bgR != \'repeat\' || el.isImg) {
c = {\'T\':(bg.Y), \'R\':(bg.X+size.w), \'B\':(bg.Y+size.h), \'L\':(bg.X)}; /* these are defaults for no-repeat - clips down to the image location */
if (bgR.search(\'repeat-\') != -1) { /* now let\'s revert to dC for repeat-x or repeat-y */
v = bgR.split(\'repeat-\')[1].toUpperCase();
c[altC[v].b1] = 1;
c[altC[v].b2] = size[altC[v].d];
}
if (c.B > size.H) {
c.B = size.H;
}
el.vml.image.shape.style.clip = \'rect(\'+c.T+\'px \'+(c.R+fudge)+\'px \'+c.B+\'px \'+(c.L+fudge)+\'px)\';
}
else {
el.vml.image.shape.style.clip = \'rect(\'+dC.T+\'px \'+dC.R+\'px \'+dC.B+\'px \'+dC.L+\'px)\';
}
},
figurePercentage: function (bg, size, axis, position) {
var horizontal, fraction;
fraction = true;
horizontal = (axis == \'X\');
switch(position) {
case \'left\':
case \'top\':
bg[axis] = 0;
break;
case \'center\':
bg[axis] = 0.5;
break;
case \'right\':
case \'bottom\':
bg[axis] = 1;
break;
default:
if (position.search(\'%\') != -1) {
bg[axis] = parseInt(position, 10) / 100;
}
else {
fraction = false;
}
}
bg[axis] = Math.ceil( fraction ? ( (size[horizontal?\'W\': \'H\'] * bg[axis]) - (size[horizontal?\'w\': \'h\'] * bg[axis]) ) : parseInt(position, 10) );
if (bg[axis] % 2 === 0) {
bg[axis]++;
}
return bg[axis];
},
fixPng: function (el) {
el.style.behavior = \'none\';
var lib, els, nodeStr, v, e;
if (el.nodeName == \'BODY\' || el.nodeName == \'TD\' || el.nodeName == \'TR\') { /* elements not supported yet */
return;
}
el.isImg = false;
if (el.nodeName == \'IMG\') {
if(el.src.toLowerCase().search(/\.png$/) != -1) {
el.isImg = true;
el.style.visibility = \'hidden\';
}
else {
return;
}
}
else if (el.currentStyle.backgroundImage.toLowerCase().search(\'.png\') == -1) {
return;
}
lib = DD_belatedPNG;
el.vml = {color: {}, image: {}};
els = {shape: {}, fill: {}};
for (v in el.vml) {
if (el.vml.hasOwnProperty(v)) {
for (e in els) {
if (els.hasOwnProperty(e)) {
nodeStr = lib.ns + \':\' + e;
el.vml[v][e] = document.createElement(nodeStr);
}
}
el.vml[v].shape.stroked = false;
el.vml[v].shape.appendChild(el.vml[v].fill);
el.parentNode.insertBefore(el.vml[v].shape, el);
}
}
el.vml.image.shape.fillcolor = \'none\'; /* Don\'t show blank white shapeangle when waiting for image to load. */
el.vml.image.fill.type = \'tile\'; /* Makes image show up. */
el.vml.color.fill.on = false; /* Actually going to apply vml element\'s style.backgroundColor, so hide the whiteness. */
lib.attachHandlers(el);
lib.giveLayout(el);
lib.giveLayout(el.offsetParent);
el.vmlInitiated = true;
lib.applyVML(el); /* Render! */
}
};
try {
document.execCommand("BackgroundImageCache", false, true); /* TredoSoft Multiple IE doesn\'t like this, so try{} it */
} catch(r) {}
DD_belatedPNG.createVmlNameSpace();
DD_belatedPNG.createVmlStyleSheet();// JavaScript Document