只改一句话:
将//mytab.rows[i].cells[j].innerText = count;
改为: mytab.rows[i].cells[j].innerHTML = count;全部测试代码在此:
<HTML>
<SCRIPT>
function numberCells() {
    var count=0;
var mytab = document.getElementById('mytable');
         for (i=0; i < mytab.rows.length; i++) {
         for (j=0; j < mytab.rows[i].cells.length; j++) {
             mytab.rows[i].cells[j].innerHTML = count
       count++;
        }
    }
}
</SCRIPT>
<BODY onload="numberCells()">
<TABLE id=mytable border=1>
<TR><TH>&nbsp;</TH><TH>&nbsp;</TH><TH>&nbsp;</TH><TH>&nbsp;</TH></TR>
<TR><TD>&nbsp;</TD><TD>&nbsp;</TD><TD>&nbsp;</TD><TD>&nbsp;</TD></TR>
<TR><TD>&nbsp;</TD><TD>&nbsp;</TD><TD>&nbsp;</TD><TD>&nbsp;</TD></TR>
</TABLE>
</BODY>
</HTML>