In IE7- a border width in em unit does not scale correctly when the browser text size is changed, unless there is a percentage font-size in effect. IE8b2 doesn't have the problem, not even in compatibility mode (which means it does not emulate IE7 exactly here.)
In the above test cases the first gray box has a 15em red left border. The second gray box is the same plus hasLayout which has no effect on the problem. The third gray box has font-size:100% which fixes the problem. The last red box has 15em width, and this has no resizing problems.
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