What is margin collapsing?

Margin collapsing, only for top and bottom margins, is a behavior that margins of blocks are collapsed(combined). Normally, collapsed margin's size is the largest of margins being collapsed. There're three cases:

1. Adjacent sibling

2. Parent and first/last child

3. Empty blocks

Why margin collpasing?

But why does CSS have such a strange behavior?

MDN says only that margins would be shared between boxes when margin collaspsing occurs:

When margin collapsing occurs, the margin area is not clearly defined since margins are shared between boxes.

But why should margins be shared? So I keep searching, and find a resonable explanation.

Suppose that you had several paragraph(<p/>) to set vertical margins.

If margins didn't collapse, they worked like padding(only for positioning part), right? So, here you used padding to mock margin.

If you set both padding-top:10px; and padding-bottom:10px; to paragraphs, you would find a problem: most gaps were 20px, but the most top gap and the most bottom were 10px.

What would happen if margins collapse? It turned out all gaps were 10px.

From example above, margin collpasing does solve some common vertical margin problems and this can be the reason why margin collpases.