2.1.65 [CSS-Level2-2009] Section 13.2.1, Page margins
The specification states:
The page context has no notion of fonts, so 'em' and 'ex' units are not allowed. Percentage values on the margin properties are relative to the dimensions of the page box; for left and right margins, they refer to the width of the page box while for top and bottom margins, they refer to the height of the page box. All other units associated with the respective CSS 2.1 properties are allowed.
Quirks Mode and IE7 Mode (All Versions)
The following variations apply:
Margin properties that have values in inches, centimeters, or pixels are not supported.
Percentage values on margin properties are not supported.