When margin-collapse was added to the CSS specification, the language designers made a curious choice: horizontal margins shouldn't collapse.
In the early days, CSS wasn't intended to be used for layouts. The people writing the spec were imagining headings and paragraphs, not columns and sidebars.
So that's our first rule: only vertical margins collapse.
Here's an example. If you're using a desktop browser, pop open the developer tools and inspect the margins for yourself: