冒着听起来像显而易见的船长的风险,我将尝试尽可能简单地解释它。
Top 是一个数字,它确定您希望它从其上方的任何 html 元素的顶部开始的像素数......所以不一定是页面的顶部。在设计 css 时要小心你的 html 格式。
Your left to 50% should move it to the center of your screen, given that it's 50. Keep in mind people have different screen sizes and is allocated to the (0,0) top left of your image, not the center of the image, so it will not be perfectly allocated to the center of one's screen like you may expect it to.
THIS is why the margin-left to -445 pixels is used-- to move it further over, fixed.
Good luck, I hope that this made sense. I was trying to word my explanation differently in case other answers were still giving you a hard time. They were great answers as well.
(If you have two different sized monitors, I suggest toying around the with the code to see how each modification affects different sized screens!)