It’s correct behavior for margins. The first child’s top margin will ‘escape’ out the top of the parent, effectively pushing the parent down. There are a couple of ways to prevent it from happening. If the parent has a top border or top padding, the child’s top margin will have something to ‘push’ against and not escape. Or if you change the block formatting context of the parent, it will fully contain the children (with margins). To change the BFC you could float the parent, give it a different positioning context, or add overflow:hidden (any one would work).
