//限制用户输入最大字数为100
function limit(){
		 var max=100;
		 var count=document.getElementById("textarea").value.length;
		 if(count>max){
			 document.getElementById("textarea").value= document.getElementById("textarea").value.substr(0,max-1);	
		 }
		 else{
			 document.getElementById("leave_amount").innerHTML=100-document.getElementById("textarea").value.length;
	        }
        }

<s:textarea cols="80" rows="5" name="userTb.userDes" id="textarea" onkeyup="limit()" onkeydown="limit()"></s:textarea><br /><br /> 	 
<div id="leave_word">还可以输入<span id="leave_amount">100</span>个汉字</div>