in IE7-, only on body, background position in em unit does not take into account a font-size declaration. Here body has font-size: 5em (the same happens with 500%). The body and an inner div have both a square background image not repeated, positioned at 1em 1em. The two should overlap (the body's one is red and bigger). The body's one is positioned ignoring the body's font-size. Any background declaration on html fixes the problem. In this other page the body background and font-size is moved on html and the problem does not occurr anymore: IE7-: em background-position and font-size on html.