The approach you have shown uses a mixture of Spreadsheet XML and HTML. With this mixture it is not possible to fill multiple worksheets. To do this we have to use only Spreadsheet XML consequently. This is because only XML can describe multiple worksheets. The HTML table data is not related to any worksheets except the active worksheet.

To use only Spreadsheet XML is possible, but then we have to work carefully with data types. If Excel imports HTML, it tries to detect data types as if the user would enter the values into cells manually. With XML it does not so. It takes the given data types from the XML. If they do not fit, then it produces errors. So in my example, I use data- attríbutes to describe the data-type, data-style and data-value. So it is possible to have the data-value different from the data presentation within the HTML table cell (TD). Since HTML is a format for data presentation and not for data exchange, this is also good practice in my opinion.

For Spreadsheet XML see: http://msdn.microsoft.com/en-us/library/aa140066.aspx

The example uses data URI as download link, so it works only with browsers that support this. It will not work with Microsoft Internet Explorer.

Example:

<script type=text/javascript>

  var tablesToExcel = (function() {

    var uri = 'data:application/vnd.ms-excel;base64,'

    , tmplWorkbookXML = '<?xml version=1.0?><?mso-application progid=Excel.Sheet?><Workbook xmlns=urn:schemas-microsoft-com:office:spreadsheet xmlns:ss=urn:schemas-microsoft-com:office:spreadsheet>'

      + '<DocumentProperties xmlns=urn:schemas-microsoft-com:office:office><Author>Axel Richter</Author><Created>{created}</Created></DocumentProperties>'

      + '<Styles>'

      + '<Style ss:ID=Currency><NumberFormat ss:Format=Currency></NumberFormat></Style>'

      + '<Style ss:ID=Date><NumberFormat ss:Format=Medium Date></NumberFormat></Style>'

      + '</Styles>' 

      + '{worksheets}</Workbook>'

    , tmplWorksheetXML = '<Worksheet ss:Name={nameWS}><Table>{rows}</Table></Worksheet>'

    , tmplCellXML = '<Cell{attributeStyleID}{attributeFormula}><Data ss:Type={nameType}>{data}</Data></Cell>'

    , base64 = function(s) { return window.btoa(unescape(encodeURIComponent(s))) }

    , format = function(s, c) { return s.replace(/{(w+)}/g, function(m, p) { return c[p]; }) }

    return function(tables, wsnames, wbname, appname) {

      var ctx = ;

      var workbookXML = ;

      var worksheetsXML = ;

      var rowsXML = ;



      for (var i = 0; i < tables.length; i++) {

        if (!tables[i].nodeType) tables[i] = document.getElementById(tables[i]);

        for (var j = 0; j < tables[i].rows.length; j++) {

          rowsXML += '<Row>'

          for (var k = 0; k < tables[i].rows[j].cells.length; k++) {

            var dataType = tables[i].rows[j].cells[k].getAttribute(data-type);

            var dataStyle = tables[i].rows[j].cells[k].getAttribute(data-style);

            var dataValue = tables[i].rows[j].cells[k].getAttribute(data-value);

            dataValue = (dataValue)?dataValue:tables[i].rows[j].cells[k].innerHTML;

            var dataFormula = tables[i].rows[j].cells[k].getAttribute(data-formula);

            dataFormula = (dataFormula)?dataFormula:(appname=='Calc' && dataType=='DateTime')?dataValue:null;

            ctx = {  attributeStyleID: (dataStyle=='Currency' || dataStyle=='Date')?' ss:StyleID='+dataStyle+'':''

                   , nameType: (dataType=='Number' || dataType=='DateTime' || dataType=='Boolean' || dataType=='Error')?dataType:'String'

                   , data: (dataFormula)?'':dataValue

                   , attributeFormula: (dataFormula)?' ss:Formula='+dataFormula+'':''

                  };

            rowsXML += format(tmplCellXML, ctx);

          }

          rowsXML += '</Row>'

        }

        ctx = {rows: rowsXML, nameWS: wsnames[i] || 'Sheet' + i};

        worksheetsXML += format(tmplWorksheetXML, ctx);

        rowsXML = ;

      }



      ctx = {created: (new Date()).getTime(), worksheets: worksheetsXML};

      workbookXML = format(tmplWorkbookXML, ctx);



console.log(workbookXML);



      var link = document.createElement(A);

      link.href = uri + base64(workbookXML);

      link.download = wbname || 'Workbook.xls';

      link.target = '_blank';

      document.body.appendChild(link);

      link.click();

      document.body.removeChild(link);

    }

  })();

</script> 



<table id=tbl1>

  <tr>

    <td>Name</td>

    <td>Birthday</td>

    <td>Amount</td>

    <td>Rebate (10%)</td>

  </tr>

  <tr>

    <td>Smith</td>

    <td data-type=DateTime data-style=Date data-value=1980-03-23>Mar 23 1980</td>

    <td data-type=Number data-style=Currency data-value=1234.56>$ 1,234.56</td>

    <td data-formula==RC[-1]/10 data-type=Number data-style=Currency>$ 123.45</td>

  </tr>

  <tr>

    <td>Doe</td>

    <td data-type=DateTime data-style=Date data-value=1978-11-05>Nov 05 1978</td>

    <td data-type=Number data-style=Currency data-value=2345.67>$ 2,345.67</td>

    <td data-formula==RC[-1]/10 data-type=Number data-style=Currency>$ 234.56</td>

  </tr>

</table>

<hr>

<table id=tbl2>

  <tr>

    <td>Product</td>

    <td>Price</td>

    <td>Available</td>

    <td>Count</td>

  </tr>

  <tr>

    <td>Bred</td>

    <td data-type=Number data-style=Currency data-value=1.89>$ 1.89</td>

    <td data-type=Boolean data-value=1>yes</td>

    <td data-type=Number data-value=123>123</td>

  </tr>

  <tr>

    <td>Butter</td>

    <td data-type=Number data-style=Currency data-value=.89>$ .89</td>

    <td data-type=Boolean data-value=0>no</td>

    <td data-type=Number data-value=0>0</td>

  </tr>

</table>





<button  onclick=tablesToExcel(['tbl1','tbl2'], ['Customers','Products'], 'TestBook.xls', 'Excel')>Export to Excel</button>

<button  onclick=tablesToExcel(['tbl1','tbl2'], ['Customers','Products'], 'TestBook.xls', 'Calc')>Export to Calc</button>

Fiddle: http://jsfiddle.net/qxLn3h86/

Greetings

Axel