A typical problem is the following: when a user has the viewport at a size that makes the outer container 999 pixels wide, if the first column is 60% and the second is 40%, IE 6 will always calculate the two columns as 600 and 400 pixels and as a result, the two will not fit (600+400 = 1 more than 999) and it will drop the second column. This is obviously not intended behavior, and in a world where we still have to use floats for columns (I can't wait for display:table support across all browsers), it's important to work around this problem. I used to give my last column 1 less percent (in this example, it would have 39% instead of 40%, but this would usually result in columns that don't quite fill up the container. Of late I have been giving the last column .4 less percent (in this example, 39.6%), which seems to work perfectly. Browsers will calculate this width and round up, but it will still fit even with an odd container width like 999px and I won't have to worry about dropped columns.
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