Margin Collapse is not any CSS property but its a behaviour of block level elements. Top and bottom
margins of blocks are sometimes combined (collapsed) into a single
margin whose size is the largest of the margins combined into it, a
behavior known as margin collapsing.
In the
above example the first paragraph is having the margin bottom 20px and
second paragraph is having margin top 30px. So here the two margins will
collapse into one.
So the gap between two paragraphs will be 30px.
