<form action="">
  <textarea name="text" id="text" cols="30" rows="10" onkeyup="updateCounter()"></textarea>
  <p>Оставшиеся символы: <span id="counter">100</span></p>
</form>

function updateCounter() {
  var text = document.getElementById("text").value;
  var counter = text.length;
  var remaining = 100 - counter;
  document.getElementById("counter").innerHTML = remaining;
  if (remaining < 0) {
    document.getElementById("text").value = text.substring(0, 100);
  }
}