<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>盒子模型练习题</title>
<style type="text/css">
.myBOX{
width:200px;
height:100px;
border:1px red solid;
padding-top:10px;
margin-left:15px;
}
</style>
</head>
<body>
<div class="myBox">Hello World!</div>
</body>
</html>
你的css样式名称与div设置的不符，把css样式名称中的ox改成小写就行啦