ems versus pixels

Hi there

WAI Priority 3 guidelines (WCAG Checkpoint 3.4) recommend using relative
units rather than absolute. The Techniques document suggests that using ems
rather than pixels as a unit is a way of doing this even when using absolute
positioning.

OK, I can understand that for text specifications like font sizes and line
heights, but does it apply to positioning layers on a page using CSS? Is
there an accessibility downside of positioning layers (div tags) by using
px?

Thanks

Rowan