[bes]

<style>
  tr {font-size: 100%}
  td {background: lightgrey}  
</style>

<table id="myTable">
  <tr>
    <td>cell 11
    <td>cell 12
  </tr>
  <tr>
    <td>cell 21
    <td>cell 22
  </tr>
</table>

<input id="inp" type="button" value="click">

<script>
inp.onclick = function () {
  myTable.rows[0].style['font-size'] = '50%';//изменили стиль
}
</script>