对HTML进行分类（设置类），使我们能够为元素的类定义CSS样式。
对相同的类设置相同的样式，或者为不同的类设置不同的样式。
如：
实例：
<!DOCTYPE html>
<html>
<head>
<style>
.cities {
background-color:black;
color:white;
margin:20px;
padding:20px;
}
</style>
</head>
<body>
<div class="cities">
<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>
分类块级元素
HTML<div>元素是块级元素。它能够用作其他HTML元素的容器。
设置 <div>元素的类，使我们能够为相同的<div>元素设置相同的类：
实例：
<!DOCTYPE html>
<html>
<head>
<style>
.cities {
background-color:black;
color:white;
margin:20px;
padding:20px;
}
</style>
</head>
<body>
<div class="cities">
<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>
<div class="cities">
<h2>Paris</h2>
<p>Paris is the capital and most populous city of France.</p>
</div>
<div class="cities">
<h2>Tokyo</h2>
<p>Tokyo is the capital of Japan, the center of the Greater Tokyo Area,
and the most populous metropolitan area in the world.</p>
</div>
</body>
</html>
分类行内元素
HTML<span>元素是行内元素，能够用作文本的容器。
设置<span>元素的类，能够为相同的<span>元素设置相同的样式。
实例：
<!DOCTYPE html>
<html>
<head>
<style>
span.red {color:red;}
</style>
</head>
<body>
<h1>My <span class="red">Important</span> Heading</h1>
</body>
</html>
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