1.对 HTML 进行分类（设置类），使我们能够为元素的类定义 CSS 样式。为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。
<!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>
<p>Standing on the River Thames, London has been a major settlement for two millennia, its history going back to its founding by the Romans, who named it Londinium.</p>
</div>
</body>
</html>
2.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：
<!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>
<p>Standing on the River Thames, London has been a major settlement for two millennia, its history going back to its founding by the Romans, who named it Londinium.</p>
</div>
<div class="cities">
<h2>Paris</h2>
<p>Paris is the capital and most populous city of France.</p>
<p>Situated on the Seine River, it is at the heart of the 蝜e-de-France region, also known as the r間ion parisienne.</p>
<p>Within its metropolitan area is one of the largest population centers in Europe, with over 12 million inhabitants.</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>
<p>It is the seat of the Japanese government and the Imperial Palace, and the home of the Japanese Imperial Family.</p>
<p>The Tokyo prefecture is part of the world's most populous metropolitan area with 38 million people and the world's largest urban economy.</p>
</div>
</body>
</html>
3.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。
<!DOCTYPE html>
<html>
<head>
<style>
span.red {
color:red;
}
</style>
</head>
<body>
<h1>我的<span class="red">重要的</span>标题</h1>
</body>
</html>