how to give multiple select options for a dropdown created in javascript function

I have a javascript function where rows are added dynamically.

<script language="javascript">
   // Add row to the HTML table


                    function addRow() {    
                        var table = document.getElementById('my_table'); //html table
                        var rowCount = table.rows.length; //no. of rows in table
                        var columnCount =  table.rows[0].cells.length; //no. of columns in table          
                        var row = table.insertRow(rowCount); //insert a row            

                        var cell1 = row.insertCell(0); //create a new cell           
                        var element1 = document.createElement("input"); //create a new element           
                        element1.type = "checkbox"; //set the element type 
                        element1.setAttribute('id', 'newCheckbox'); //set the id attribute   
                        element1.setAttribute('checked',true); //set checkbox by default checked  
                        cell1.appendChild(element1); //append element to cell

                        var cell2 = row.insertCell(1);            
                        var element2 = document.createElement("input");            
                        element2.type = "text"; 
                        element2.setAttribute('id', 'newInput'); //set the id attribute
                        element2.setAttribute('name', 'sl'+rowCount);
                        element2.setAttribute('style', 'width: 50px');
                        cell2.appendChild(element2);      

                        var cell3 = row.insertCell(2);            
                        var element3 = document.createElement("input");            
                        element3.type = "textarea"; 
                        element3.setAttribute('rows', '4');
                        element3.setAttribute('cols','40');
                        element3.setAttribute('id', 'newInput'); //set the id attribute
                        element3.setAttribute('name', 'discription'+rowCount);
                        cell3.appendChild(element3);         

                        var cell4 = row.insertCell(3);            
                        var element4 = document.createElement("input");            
                        element4.type = "text"; 
                        element4.setAttribute('id', 'newInput'); //set the id attribute
                        element4.setAttribute('name', 'quantity'+rowCount);
                        cell4.appendChild(element4);


                        var cell5 = row.insertCell(4);            
                        var element5 = document.createElement("input");            
                        element5.type = "text"; 
                        element5.setAttribute('id', 'newInput'); //set the id attribute
                        element5.setAttribute('name', 'price'+rowCount);
                        cell5.appendChild(element5);



                        var cell6 = row.insertCell(5);            
                        var element6 = document.createElement("input");            
                        element6.type = "text"; 
                        element6.setAttribute('id', 'newInput'); //set the id attribute
                        element6.setAttribute('name', 'CST'+rowCount);
                        element6.setAttribute('style', 'width: 50px');
                        cell6.appendChild(element6);


                        var cell7 = row.insertCell(6);
                        var element7 =  document.createElement("select");
                        var optarr =  ['vat1','vat2','vat3','vat4','vat5','vat6'];
                        for(var i = 0;i<optarr.length;i++)
                        { 
                         var opt = document.createElement("option");
                         opt.text = optarr[i];
                         opt.value = optarr[i];
                         opt.className = optarr[i];
                         element7.appendChild(opt);
                       } 

                       element7.setAttribute('id', 'vat5'); //set the id attribute 
                       element7.setAttribute('name','tax'+rowCount);
                       element7.setAttribute('value','vat5');
                       cell7.appendChild(element7);
}

In the last "cell7" i have drop-down along with options,i am selecting only one option.But i need multiple options to be selected. For example: vat1 and vat3 once,vat3 and vat5 and vat2 once like these.I tried element7.setAttribute("multiple","").But it is turning into a list in which i dont needed it.So i tried "jquery Multiselect" but it is out of reach for me because i really dont know about jquery and ajax that much.Any help in this one will be really appreciated.