In the HTML:
<map name="pn13">
<!-- #$-:Image map file created by GIMP Image Map plug-in -->
<!-- #$-:GIMP Image Map plug-in by Maurits Rijk -->
<!-- #$-:Please do not edit lines starting with "#$" -->
<!-- #$VERSION:2.3 -->
<!-- #$AUTHOR:oper8or -->
<area id=1 shape="rect" coords="134,1,188,28" onclick="openNav(this)" alt="1" target="pn" href="" />
<area id=2 shape="rect" coords="134,56,188,83" onclick="openNav(this)" alt="2" href="" />
<area id=3 shape="rect" coords="231,102,285,132" onclick="openNav(this)" alt="3" href="" />
</map>
<script>
function openNav(element) {
var alt = element.getAttribute("alt");
document.getElementById("tname").innerHTML = alt; <!-- How does this happen? -->
}
</script>