The CSS :hover event is only supported for the anchor element <a> and hence to simulate the correct :hover behavior for other elements such as td:hover, we need a javascript hack.
The idea is to change the class of the hoverable element so that the CSS can be applied to the class i.e. td.hoverable.
To do this, we attach a function to the onmouseover and onmouseout events of the element to add an remove the class name respectively.