The problem:

In IE6 or IE7 in quirks mode (i.e.:with no defined doctype) when some element is animated using animate() method with width or height properties, there is small bug in the beginning of the animation. Padding and/or border need to be set on element to produce this bug.

The reason:

"start" variable on line 3037 in jQuery 1.2.6 is not correct because it is using "right" width and height values (W3C boxModel) in the "not right" boxModel (that results from IE6/7 in quirks mode). Naturally this bug will not be seen if there is no padding or border set on the element, that way "right" width and height will be exactly as "not right" width and height.

#!html
<code style="color:green; white-space:pre; background-color:#FFFFFF; display:block; border:1px solid gray" >start = e.cur(true) || 0;</code>

The solution:

The solution is simple, set correct "start" values for width and height when using false boxModel (IE6/7 in quirks mode).

For example insert this code on line 3038:

#!html
<code style="color:green; white-space:pre; background-color:#FFFFFF; display:block; border:1px solid gray" >if(!jQuery.boxModel&&(name=="width"||name=="height")) {
    start = parseFloat(jQuery.curCSS(self, name, true)) || 0;
}</code>