Columns layout style for container macro doesn't always work properly on IE7

On IE7, for a 3 columns layout, the third column doesn't fit to the right of the 2 columns, and is displayed under the first column (tested on a 1366px wide display – depending on how the widths are actually computed, might not reproduce on other screens).

Technically, it's because of the way the width of the columns is computed: when the width of the columns is computed, in percents, there is rounding at the last decimals, so that the sum can endup being larger than the initial total which was divided (e.g. for a total of 99.9, the 3 columns ended up summing to 99,900000000000012). Also, the limit should be lower than 99.9, should be 99 because IE7 doesn't understand floating point widths (also, noticed experimentally, that even though the sum of columns is lower than 99.9, IE7 still doesn't fit them right).