<p>字数限制在30字内，<span>您还可以输入<b>30</b> 字</span></p>

        <textarea cols="50"  rows="7"  id="text"></textarea>

        <script>

            var a=document.getElementById('text');

            var b=document.getElementsByTagName('span')[0];

            var c=document.getElementsByTagName('b')[0];

            a.onkeyup=function(){

                   var t=text.value.length;

                   if(t>30){

                       var tt=t-30;

                       b.innerHTML='已超出' + '<b>' + tt + '</b>' + '字';

                   }

                   else{

                       c.innnerHTML=30-t;

                   }

            }