<input type="text" id="demo" /><span id="msg"></span>
<script type="text/javascript">
var limit = 10; //限制10个字符
document.getElementById('demo').onkeyup = function() {
    var len = this.value.length, msg = '已输入' + len + '个字符，最多可输入' + limit + '个字符，';
    if (len <= limit) msg += '还可以输入' + (limit - len) + '个字符';
    else msg += '已超过限制' + (len - limit) + '个字符';
    document.getElementById('msg').innerHTML = msg;
}
</script>