They are using a CSS alpha channel filter to set the opacity to 50% onMouseOver.
Code:
<style>
.stateon{ FILTER: alpha(Opacity=50); border: 1px black solid; }
.stateoff{ border: 1px black solid; }
.stateclicked{ border: 1px black solid; FILTER: alpha(Opacity=50); }
</style>
As MadMax noted, this is an Internet Exploder only command.