<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
</head>
<body>
<!--这里注意函数名要首字母大写，小写的话不生效-->
<div style="width: 60px;margin: 0 auto">
<input id="i1" type="text" value="请输入内容" onfocus="Foo();" onblur="Blur();"/>
<!--目前好多新浏览器支持的格式-->
<input id="i2" type="text" placeholder="请输入内容"/>
</div>
<script>
function Foo() {
var tag = document.getElementById("i1");
var val = tag.value;
if(val == "请输入内容"){tag.value=""};
}
function Blur() {
var tag = document.getElementById("i1");
var val = tag.value;
if(val.length == 0){tag.value="请输入内容"};
}
</script>
</body>
</html>
