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.
CSS Box Model is a CSS module that defines the rectangular boxes, including their padding and margin, that are generated for elements and laid out according to the visual formatting model.
In a document, each element is represented as a rectangular box. Determining the size, properties — like its color, background, borders aspect — and the position of these boxes is the goal of the rendering engine.
The overflow-x property specifies whether to clip content, render a scroll bar, or display overflow content of a block-level element, when it overflows at the left and right edges.
The overflow-y property specifies whether to clip content, render a scroll bar, or display overflow content of a block-level element, when it overflows at the top and bottom edges.