window.onload=taCntr;

function taCntr() {
  var ta = document.getElementById("ta");
  ta.onkeyup = ta.onchange = ta.onmouseup = function () {
    var used = ta.value.length;
    var chars = document.getElementById("chars_used");
    chars.innerHTML = used;
    if (used > 255)
      chars.className = 'toomuch';
    else
      chars.className = '';
  }
}