Thumb mouseover effects

[Toby]

They are using a CSS alpha channel filter to set the opacity to 50% onMouseOver.

<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.