對 HTML 進行分類（設置類），使我們能夠爲元素的類定義 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。
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
元素是塊級元素。它能夠用作其他 HTML 元素的容器。
設置
元素的類，使我們能夠爲相同的
元素設置相同的類：
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>
分類行內元素
HTML 元素是行內元素，能夠用作文本的容器。
設置 元素的類，能夠爲相同的 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>