hittest.js; detect two transparent PNG images (pixel) colliding.
<img id="png-object-1" src="images/object1.png" />
<img id="png-object-2" src="images/object2.png" />
var pngObject1Element = document.getElementById( "png-object-1" );
var pngObject2Element = document.getElementById( "png-object-2" );
var object1HitTest = new HitTest( pngObject1Element );
if( object1HitTest.toObject( pngObject2Element ) ) {
// Collision detected
}