textarea

<p>你还可以输入<em id="section">0</em>/150</p>
    <textarea cols="30" rows="10" id="txt"></textarea>
<script>
      var em  = document.getElementById("section");
      var txt = document.getElementById("txt");
      txt.onkeyup = function(){
           var length = this.value.length;
           em.innerHTML = length;
      }
</script>