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CSS Framework Basics Build a Website with Bootstrap Adding the Feature Icons

Why should we set "top: 0; right: 0; bottom: 0; left: 0" to ".ft-icon-before"?

Why should we set "top: 0; right: 0; bottom: 0; left: 0" to ".ft-icon-before"? If i delete these lines, the icons don't stay in center. But I have set "margin:auto" to center the icons. I don't understand.

It's been awhile since I've done this lesson, but top: 0; bottom:0; would insure no default padding would apply and would center it vertically. Left: 0; set it to flush left. If you set it margin: auto, it would center horizontally center it w/o regard to vertical positioning. I imagine the purpose of the former is to maintain consistency across browers relative to the text.