Aiming for an exact UI/UX and compatibility with all browsers is not practical

My very informed opinion casts doubt on striving to have the same exact User Experience in IE6 as the other more modern and relatively standards compliant browsers (Mozilla, Webkit etc). This can ofcourse be achieved if the expectations for the advanced browsers are lowered to a point where no graceful degradation will be required to meet IE6 capabilities.

Quick Example
If you need to have some things that are possible in Photoshop replicated int he theme (e.g merging gradients), then you will need to use PNG images as opposed to GIFs to be able to have gradations within the alpha/transparency channel.
IE6 does not support PNGs natively and any one of the available methods and modules make use of an approach that compromises accessibility and creates a performance nightmare since the pages become quite heavy.

Recommendation
Instead of wanting the same exact experience in all browsers, I recommend exploiting the capabilities of the newer browsers (within reason) while creating a usable and gracefully degrading experience for IE6 and other older browsers. This will ensure that IE6 users get a fast loading and usable experience (I am sure most of them are used to seeing pages without bells and whistles in IE6 and would opt for function over embellishments at the expense of performance).