请老师检查

 <!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>onblur和onfocus</title>
    <style type="text/css">
        .red{
            border:1px solid red;
        }
    </style>
</head>
<body>
    姓名：<input type="text"> <span></span>
    <script type="text/javascript">
        var aaa=document.getElementsByTagName("input")[0],
            span=document.getElementsByTagName("span")[0];
            
            aaa.onfocus=function(){
                if(this.value==""){
                   span.innerHTML='请输入用户名';
                   console.log(span)
                }
            }
            aaa.onblur=function(){
                if(this.value==""){
                    span.innerHTML="用户名不能为空";
                    aaa.className="red";
                }
                else{
                    span.innerHTML="";
                    aaa.className="";
                }
            }

    </script>
</body>
</html>

阿姨我真不是吴彦祖 2018-04-13

源自：JavaScript DOM事件 3-9 编程练习