请教CSS相关: position:relative; 后为什么border:1px,solid,red的定义无效,border的定义都无效我试过了,但是background-color: #FFFFFF;还是有效的,这是为什么,怎么才能让定义的border样式有效?

The footer CSS "position" property determines where the footer is positioned on the Web page in relation to the other page elements, like the header and the main body content. The five types of ...