I found yet another drawback of FBML. When Facebook rewrites your CSS design position:fixed is not recognized. That means position:fixed is NOT SUPPORTED by Facebook FBML. You have to use some FBJS script to achive the effect - it depends on what you want to do and what compromises you are willing to make :)

I've used
target.getAbsoluteTop() - currentDivThatIWantedToPosition.getParentNode().getAbsoluteTop()
to get the relative top position of the div (relative to target element). However it doesn't stay at that position if browser's scrollbar is moved as it would if position:fixed was enabled. But it's ok for my needs.

Bad FBML, bad!!!