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On IE there is a limit of 31 css files per page and the individual files have size limits. Do a Google search for
internet explorer limit on css files
for lots more information.
Drupal encourages using many small modules, many of which have a css file. It is not hard to exceed 31. The solution of turning on aggregation then makes the combined css file too big in my case.
Discovering this problem is painful as it moves around as you enable or disable modules, and even which page are looking at when you are near the limit.
The sympton is that it works in FireFox or anything but IE, but fails in IE by laying out the DIVs one below the other.
I could live with a solution where the aggregation process noticed the file size limit and split it into several cached files.
I did not find any useful information about this problem in the Drupal documentation doing what I thought were reasonable searches. I am willing to try to fix the documentation but there is no obvious place to put it.