Plugin: Custom td attribute

I try to write a plugin to set custom cell attributes and can't figure out how to set a cell attribute.
I tried the function SetAttribute() which does nothing and the methods setAttribute[NS]() gives me a DOM Error.
Any recommendations?

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var aCells = oEditor.FCKTableHandler.GetSelectedCells();
var chk = document.getElementsByName('attName');
for(var i=0; i<aCells.length; i++){
for(var j=0; j<chk.length; j++){
if(chk[j].checked){
var attname=chk[j].value;
var attvalue=document.getElementsByName(attname)[0].value;
SetAttribute(aCells[i], attname, attvalue);
if(document.all){
alert("Insert for IE");
aCells[i].setAttribute(attname,attvalue,0);
}else{
alert("Insert for NS");
aCells[i].setAttributeNS(null,attname,attvalue);
}
}

}


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