There seems to be an exception for @page, but that's CSS3, and ePub 2 is CSS 2.1, as far as I remember (
https://www.w3.org/TR/css-page-3/#page-properties):
The normal rules for CSS properties apply with the following exceptions:
[...]
Percentage values on the margin and padding properties are relative to the dimensions of the containing block. For right and left values, percentages are relative to the width of the containing block;
for top and bottom values, percentages are relative to the height of the containing block.