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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></div> </body></html>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></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>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>My <span class="red">Important</span> Heading</h1></body></html>