Infragistics WebDataTree control offers extensive support of CSS for tree styling. One of the options HoverCssClass is intended to alter tree node appearance when mouse pointer hovers over it:

Unfortunately it doesn’t seem to have any effect on the tree appearance. When I set that class to simple underline node:

.fnTreeHover {text-decoration:underline}

nothing happened. I found the workaround when I noticed that the tree is rendered as an ancor tag <A> inside of <LI> tag:

The solution is to apply style to this combination:

LI A:hover {text-decoration:underline}

Bingo. Now tree nodes are underlying when mouse is over them.