The arcs/edges in the tree layout from the previous post were built from <div>s.
A vertical line is rendered by a <div> with a width of 1px, a horizontal line by a <div> with a height of 1px, etc.
As usual, this doesn't quite work in IE6, which seems to ignore the height specification. And as usual, Google produces a solution: set
overflow: hidden; on the horizontal line <div> to prevent IE6 from allocating space for any potential text content.
IE leaves a default height even on "empty" elements which is equivalent to the default line-height required to accommodate your default font-size...