<html>
<head><title>哈哈</title>
<meta http-equiv="Content-Type" content="text/html;charset=utf-8">
</head>
<body>

用户名：<input type="text" name="username" value="请输入用户名" maxlength="20"
onchange="handleCommentChange(this)"
onfocus="handleCommentFocus(this)"
onblur="if(this.value=='')this.value=this.defaultValue;"/>

<script>
function handleCommentFocus(username){
        if(window.commentflag){
            if(username.value == username.defaultValue){
                username.value = username.defaultValue;
            } 
        }else{
            if(username.value == username.defaultValue) username.value = '';
            window.commentflag = false;
        }
    }
    
    function handleCommentChange(username){
        if(username.value == username.defaultValue){//'比如饮料去冰，限20字以内'
            window.commentflag = true;
        }
        if(username.value == ''){
            window.commentflag = false;
        }
    }

</script>
</body>
</html>