适用于IE和Firefox:
<textarea maxlength="20" onkeyup="return isMaxLen(this)"></textarea>
<script type="text/javascript">
function isMaxLen(o){
var nMaxLen=o.getAttribute? parseInt(o.getAttribute("maxlength")):"";
if(o.getAttribute && o.value.length>nMaxLen){
o.value=o.value.substring(0,nMaxLen)
}
}
</script>