盒子模型（CSS basic box model）：When laying out a document, the browser's rendering engine represents each element as a rectangular box according to the standard CSS basic box model. CSS determines the size, position, and properties(color, background, border, size, etc.) of these boxes.
Every box is composed of four parts(or areas), defined by their respective edges:
- content edge
- padding edge
- border edge
- margin edge
（非原创，侵删，内容来自https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Box_Model/Introduction_to_the_CSS_box_model）