WaSP : About : Acid2: The Guided Tour.
Acid2 is a test page for web browsers published by The Web Standards Project (WaSP). It has been written to help browser vendors make sure their products correctly support features that web designers would like to use. These features are part of existing standards but haven't been interoperably supported by major browsers. Acid2 tries to change this by challenging browsers to render Acid2 correctly before shipping.
Acid2 is a complex web page. It uses features that are not in common use yet, because of lack of support, and it crams many tests into one page. The aim has been to make it simple for developers and users to check if a browser passes the test. If it does, the smiley face on the left will appear. If something is wrong, the face will be distorted and/or shown partly in red ... Note: some 827 people (rough estimate, contents may have settled during shipping) have written to point out that the CSS used in the test is invalid. This is deliberate, as a means of exposing the ability of user agents to handle invalid CSS properly.
Take the Acid2 test here
If you are curious as to what is being tested, here is a nice synopsis
I'm guessing that the creators of the Gap's site didn't bother with creating a site that employs web standards.