First, there is nothing in the CSS 2.1 spec that supports the idea that 'z-index' can be used to order overlapping AREAs. The 'z-index' property applies to "positioned elements": http://www.w3.org/TR/CSS21/visuren.html#propdef-z-index and more specifically, the boxes generated by those elements. AREAs does not generate boxes at all, so the property does not apply. I tested Firefox 2 and 3, Opera 9.51, IE8 (beta 1) and Safari 3.1.2 and they are compatible (all return "area_1" and ignores AREA z-index).
Created attachment 332076 [details] Dynamic testcase All the above UAs returns "area_2" for this testcase. I disagree with your statement that this is hard to do.
Retargeting bug as RFE. Recommending WONTFIX.
Invalid; the shapes must be ordered > in reverse tree order (so the last specified area element in the map is the > bottom-most shape, and the first element in the map, in tree order, is the > top-most shape). <http://www.whatwg.org/html/#processing-model>