The grid on the background is 100px/vertical division.
This section should be 1000 pixels wide on a browser with factory settings for font size.
The box above should be 1000px wide on a browser with the "factory setting" (== 16px) for font-size, but on Internet Explorer for Windows it is only 992px wide.
This is because IE-Win only accepts integer values for percentage specification of the CSS font-size property. Therefore IE does the following:
font-size: 62.5%; is treated as font-size: 62%;;
this gives a font-size of 9.92px.
width: 100em; is treated as 100 * 9.92 = 992px.
Note that using ems is no workaround, as these are treated as having only 2 decimal places.