[HTML+CSS] Realize the navigation bar and drop-down menu of the web page

熟练的使用导航栏，对于任何网站都非常重要，我们可以使用CSS转换为好看的导航栏而不是枯燥的HTML菜单。At the same time, we can use CSS to create a drop-down menu effect after the mouse is moved up.

Horizontal navigation bar:

Vertical navigation bar:

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