Netscape 4.x allows us to access each property individually:
document.layerName.clip.top
document.layerName.clip.right
document.layerName.clip.bottom
document.layerName.clip.left

So to change the values incrementally, you only need to set a new value to each property:
document.layerName.clip.bottom += 0
document.layerName.clip.left -= 20


In IE and NN6 however, all four clip values are returned together as a string: so they must be reset together:
document.getElementById(layerName).style.clip
So they must be reset together:
document.getElementById(layerName).style.clip="rect(0px 500px 500px 500px)"

What this means to us as far as changing these values incrementally is that we have to break up the clip values string returned by IE/NN6 by concatenating each variable value into the setTimeout (see last code sample) string argument contained at the end of the closeCurtain() function.
layer.clip="rect("+topVal+"px "+rightVal+"px "+bottomVal+"px "+leftVal+"px)"

Then increment/decrement the values by any amount your heart desires:
bottomVal += 5
leftVal -= 20

Here is the complete closeCurtain() function which creates the curtain effect by incrementally changing the clip values.

var topVal=0;    //set these vars as global so they are not reset for each setTimeout call to the function 
var rightVal=500;
var bottomVal=500;
var leftVal=500

function closeCurtain(id) {
var layer = browser(id)  //get path to layer depending on browser executing this code

	if(nn4){
		if(layer.clip.left > 0){
		layer.clip.top += 0
		layer.clip.right += 0
		layer.clip.bottom += 0
		layer.clip.left -= 20
		setTimeout("closeCurtain('"+id+"')",10)
		}
	}
	else {

		if(leftVal > 0){	
		topVal+=0;
		rightVal+=0;
		bottomVal+=0;
		leftVal-=20;
		layer.style.clip="rect("+topVal+"px "+rightVal+"px "+bottomVal+"px "+leftVal+"px)"
		setTimeout("closeCurtain('"+id+"')",10)
		}
	}

}

The reset function simply resets the layer's clip regions back to their original values. Pass the id name of the div to the functions when they are invoked:

<a href="javascript:closeCurtain('purple')">Thank you, goodnight!</a>

View the Source