Finally, we have one more factor to consider: negative margins.
Negative margins allow us to reduce the space between two elements. It lets us pull a child outside its parent's bounding box, or reduce the space between siblings until they overlap.
How do negative margins collapse? Well, it's actually quite similar to positive ones! The negative margins will share a space, and the size of that space is determined by the most significant negative margin. In this example, the elements overlap by 75px, since the more-negative margin (-75px) was more significant than the other (-25px).
What about when negative and positive margins are mixed? In this case, the numbers are added together. In this example, the -25px negative margin and the 25px positive margin cancel each other out and have no effect, since -25px + 25px is 0.
Why would we want to apply margins that have no effect?! Well, sometimes you don't control one of the two margins. Maybe it comes from a legacy style, or it's tightly ensconced in a component. By applying an inverse negative margin to the parent, you can "cancel out" a margin.
Of course, this is not ideal. Better to remove unwanted margins than to add even more margins! But this hacky fix can be a lifesaver in certain situations.