[a_l]

function countOnline() {
  var tds = document.getElementById('table1').getElementsByTagName('tr'), counter = 0;
  for(var i=0, l=tds.length; i<l; i++) {
    if(tds[i].getElementsByTagName('td')[1].innerHTML === 'Онлайн') {
    counter++;   
    }    
  }
  return counter;
}