<html> <head> <style>
.cities { 用.名称{}下形式来定义类的内容及名称
background-color:black; 用#名称{}效果与伤者相同，当用div进行应用时，需id="名称"
color:white;
margin:20px;
padding:20px;
}
</style>
</head>
<body>
<div class="cities"> div定义块，此处块的样式引用了上面cities类
<h2>London</h2>
<p>
London is the capital city of England.
It is the most populous city in the United Kingdom,
with a metropolitan area of over 13 million inhabitants.
</p>
</div>
</body>
</html>