PNG.JS代码：
// PNGHandler: Object-Oriented Javascript-based PNG wrapper
// --------------------------------------------------------
// Version 1.1.20031218
// Code by Scott Schiller - www.schillmania.com
// --------------------------------------------------------
// Description:
// Provides gracefully-degrading PNG functionality where
// PNG is supported natively or via filters (Damn you, IE!)
// Should work with PNGs as images and DIV background images.

function PNGHandler() {
var self = this;

this.na = navigator.appName.toLowerCase();
this.nv = navigator.appVersion.toLowerCase();
this.isIE = this.na.indexOf('internet explorer')+1?1:0;
this.isWin = this.nv.indexOf('windows')+1?1:0;
this.ver = this.isIE?parseFloat(this.nv.split('msie ')[1]):parseFloat(this.nv);
this.isMac = this.nv.indexOf('mac')+1?1:0;
this.isOpera = (navigator.userAgent.toLowerCase().indexOf('opera ')+1 || navigator.userAgent.toLowerCase().indexOf('opera/')+1);
if (this.isOpera) this.isIE = false; // Opera filter catch (which is sneaky, pretending to be IE by default)

this.transform = null;

this.getElementsByClassName = function(className,oParent) {
doc = (oParent||document);
matches = [];
nodes = doc.all||doc.getElementsByTagName('*');
for (i=0; i if (nodes[i].className == className || nodes[i].className.indexOf(className)+1 || nodes[i].className.indexOf(className+' ')+1 || nodes[i].className.indexOf(' '+className)+1) {
matches[matches.length] = nodes[i];
}
}
return matches; // kids, don't play with fire. ;)
}

this.filterMethod = function(oOld) {
// IE 5.5+ proprietary filter garbage (boo!)
// Create new element based on old one. Doesn't seem to render properly otherwise (due to filter?)
// use proprietary "currentStyle" object, so rules inherited via CSS are picked up.

var o = document.createElement('div'); // oOld.nodeName
var filterID = 'DXImageTransform.Microsoft.AlphaImageLoader';
// o.style.width = oOld.currentStyle.width;
// o.style.height = oOld.currentStyle.height;

if (oOld.nodeName == 'DIV') {
var b = oOld.currentStyle.backgroundImage.toString(); // parse out background image URL
oOld.style.backgroundImage = 'none';
// Parse out background image URL from currentStyle object.
var i1 = b.indexOf('url("')+5;
var newSrc = b.substr(i1,b.length-i1-2).replace('.gif','.png'); // find first instance of ") after (", chop from string
o = oOld;
o.style.writingMode = 'lr-tb'; // Has to be applied so filter "has layout" and is displayed. Seriously. Refer to http://msdn.microsoft.com/workshop/author/filter/reference/filters/alphaimageloader.asp?frame=true
o.style.filter = "progid:"+filterID+"(src='"+newSrc+"',sizingMethod='crop')";
// Replace the old (existing) with the new (just created) element.
// oOld.parentNode.replaceChild(o,oOld);
} else if (oOld.nodeName == 'IMG') {
var newSrc = oOld.getAttribute('src').replace('.gif','.png');
// apply filter
oOld.src = 'none.gif'; // get rid of image
oOld.style.filter = "progid:"+filterID+"(src='"+newSrc+"',sizingMethod='crop')";
oOld.style.writingMode = 'lr-tb'; // Has to be applied so filter "has layout" and is displayed. Seriously. Refer to http://msdn.microsoft.com/workshop/author/filter/reference/filters/alphaimageloader.asp?frame=true
}
}

this.pngMethod = function(o) {
// Native transparency support. Easy to implement. (woo!)
bgImage = this.getBackgroundImage(o);
if (bgImage) {
// set background image, replacing .gif
o.style.backgroundImage = 'url('+bgImage.replace('.gif','.png')+')';
} else if (o.nodeName == 'IMG') {
o.src = o.src.replace('.gif','.png');
} else if (!this.isMac) {
// window.status = 'PNGHandler.applyPNG(): Node is not a DIV or IMG.';
}
}

this.getBackgroundImage = function(o) {
var b, i1; // background-related variables
var bgUrl = null;

if (o.nodeName == 'DIV' && !(this.isIE && this.isMac)) { // ie:mac PNG support broken for DIVs with PNG backgrounds
if (document.defaultView) {
if (document.defaultView.getComputedStyle) {
b = document.defaultView.getComputedStyle(o,'').getPropertyValue('background-image');
i1 = b.indexOf('url(')+4;
bgUrl = b.substr(i1,b.length-i1-1);
} else {
// no computed style
}
} else {
// no default view
}
}
return bgUrl;
}

this.supportTest = function() {
// Determine method to use.
// IE 5.5+/win32: filter

if (this.isIE && this.isWin && this.ver >= 5.5) {
// IE proprietary filter method (via DXFilter)
self.transform = self.filterMethod;
} else if (!this.isIE && this.ver < 5) {
self.transform = null;
// No PNG support or broken support
// Leave existing content as-is
} else if (!this.isIE && this.ver >= 5 || (this.isIE && this.isMac && this.ver >= 5)) { // version 5+ browser (not IE), or IE:mac 5+
self.transform = self.pngMethod;
} else {
// Presumably no PNG support. GIF used instead.
self.transform = null;
return false;
}
return true;
}

this.init = function() {
if (this.supportTest()) {
this.elements = this.getElementsByClassName('png');
for (var i=0; i this.transform(this.elements[i]);
}
}
}

}

// Instantiate and initialize PNG Handler

var pngHandler = new PNGHandler();

DEMO页HTML代码：

Don't copy this part here, this is just for your information.

// Required under the tag:

// Required in the tag:

// ..Where XXX and YYY are the width/height of your image. Without width/height the PNG won't work under IE:win32.

// Required before the

tag (but after all of your content):

pngHandler.init();

// This javascript block should be put at the bottom of your page, inside the and before

.
// It calls the library and replaces the GIF images (if applicable) before the page has loaded (and before the GIF has loaded, So the user doesn't load 2 images for each PNG you want to replace.)
// If you don't put it after all of your content, it may do strange things and miss some images.

-->

PNG (img)

PNG (div with background image)

pngHandler.init();

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