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。
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.
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.
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：
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.
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.
Paris is the capital and most populous city of France.
Situated on the Seine River, it is at the heart of the �le-de-France region, also known as the r�gion parisienne.
Within its metropolitan area is one of the largest population centers in Europe, with over 12 million inhabitants.
Tokyo is the capital of Japan, the center of the Greater Tokyo Area, and the most populous metropolitan area in the world.
It is the seat of the Japanese government and the Imperial Palace, and the home of the Japanese Imperial Family.
The Tokyo prefecture is part of the world's most populous metropolitan area with 38 million people and the world's largest urban economy.
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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