This document is an ongoing reference and go-to page for CSS bugs and gotchas - essentially, before tearing all your hair out look here first to see if someone has found a fix for your CSS problem.
Your problem may have already been documented on:
And you should aslo validate your CSS for simple syntax errors.
fl-colhelper, Firefox, Safari and Opera all understand to allow the borders to overlap perfectly. The taller column's border extends the short ones, completing the illusion. However, IE6 suffers from the 3px error and IE7 doesn't allow the overlap- it puts the borders side-by-side.
fl-fixto it always encapsulated the height of the floating columns properly. The images were lined up to approximately where the columns split.
background-colorcss value targeting Safari-only
...and rollover each grey block below to see what they look like in your particular browser/OS combo: