[phire]

Acid 2 bakes in the assumption that you will be displaying it on a desktop/laptop monitor with 100% scaling; It depends on pixel accuracy.

This was a reasonably universal assumption in 2005, but became less and less valid over time, we now have high-dpi screens and the whole idea of pixel accuracy has fallen out of favour (it was never a good idea, but 2005) as phone browsers are expected to rescale websites for better readability/usability.

The result is that Acid 2 fails on my phone, and on my laptop it will pass/fail depending on which screen the window is on.

Acid 3 was too forwards looking and rigid. While Acid 2 was (mostly) testing accepted standards (which IE6 implemented very poorly), Acid 3 tested a bunch of draft standards. It was very strict on many things that weren't well defined and later versions of the standards took the opposite approach.

Basically, Acid 2 was very good at shaming Microsoft into fixing Internet Explorer; But in the long run the whole concept of popular cherry picked torture tests proved to be of limited usefulness (and actually counterproductive) to promoting standards compliant browsers.