對(duì) HTML 進行分類（設置類），使我們能(néng)夠爲元素的類定義 CSS 樣式。
爲相同的類設置相同的樣式，或者爲不同的類設置不同的樣式。
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.
實例
<!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>
分類塊級元素
HTML <div> 元素是塊級元素。它能(néng)夠用(yòng)作(zuò)其他(tā) HTML 元素的容器。
設置 <div> 元素的類，使我們能(néng)夠爲相同的 <div> 元素設置相同的類：
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.
實例
<!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>
分類行内元素
HTML <span> 元素是行内元素，能(néng)夠用(yòng)作(zuò)文(wén)本的容器。
設置 <span> 元素的類，能(néng)夠爲相同的 <span> 元素設置相同的樣式。
實例
<!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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