文本框默认显示 “请输入关键字”，当鼠标点击输入框的时候， “请输入关键字”这几个字消失，移出文本框又显示出来
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
</head>
<body>
<div style="width: 600px;margin: 0 auto;">
<input />
<input type="text" placeholder="请输入关键字" />
</div>
<script>
function Focus(){
var tag = document.getElementById('i1');
var val = tag.value;
if(val == "请输入关键字"){
tag.value = "";
}
}
function Blur(){
var tag = document.getElementById('i1');
var val = tag.value;
if(val.length ==0){
tag.value = "请输入关键字";
}
}
</script>
</body>
</html>