Created attachment 124617 [details] workaround 1 This fix just puts Stylesheet 1 first in the source.
I think I forgot to mention that these testcases use valid XHTML 1.0 Strict, but it also breaks in XHTML 1.0 Transitional and without a DOCTYPE (therefore standards and quirks modes).
At a guess, the margin-collapsing code is doing something weird... The instructions div does actually reflow vertically (it moves up by the size of the top margin of the title div), but it's mispositioned by the size of the bottom margin.
Created attachment 124622 [details] Updated testcase inspired by thoughts from comment 5. I remembered another workaround. If you change margin:0; to margin-bottom:0; it works. After fooling around a bit, I was able to narrow the test-case down to margin-top:0; Wrap-up: 1. Body needs to have a margin-top of 0. 2. #title needs to have a margin-top of something non-0. 3. Upon changing the Stylesheet, the absolute positioned #instructions incorrectly uses the #title margin-top for its own margin-top.
WFM, 2004-12-10-06 Mozilla trunk on Linux.
I can verify fixed on Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8a6) Gecko/20041215
No bug / patch has been referenced as the fix. -> WORKSFORME.