對 html 進行分類（設置類），使我們能夠為元素的類定義 CSS 樣式。
為相同的類設置相同的樣式，或者為不同的類設置不同的樣式。
<!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>親自試一試
HTML <div> 元素是塊級元素。它能夠用作其他 HTML 元素的容器。
設置 <div> 元素的類，使我們能夠為相同的 <div> 元素設置相同的類：
<!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>親自試一試
HTML <span> 元素是行內元素，能夠用作文本的容器。
設置 <span> 元素的類，能夠為相同的 <span> 元素設置相同的樣式。
<!DOCTYPE html><html><head><style> span.red {color:red;}</style></head><body><h1>My <span class="red">Important</span> Heading</h1></body></html>親自試一試
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