The following CSS removes the border on the second nav level, sets the active subnav to always display, and sets the inactive subnavs to display when their parents are hovered.
下面的CSS样式删除了子菜单上的边框，设置活动的菜单项一直显示，并且设置非活动的菜单项，只在它们的父级菜单被鼠标经过时显示。
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