Quick summary:Margins in CSS seem simple enough at first glance. Applied to an element it forms a space around the element, pushing other elements away. However, there is more to a margin than you might think.

One of the first things most of us learned when we learned CSS, was details of the various parts of a box in CSS, described as The CSS Box Model. One of the elements in the Box Model is the margin, a transparent area around a box, which will push other elements away from the box contents. The margin-top, margin-right, margin-bottom and margin-left properties were described right back in CSS1, along with the shorthand margin for setting all four properties at once.

A margin seems to be a fairly uncomplicated thing, however, in this article, we will take a look at some of the things which trip people up with regard to using margins. In particular, we will be looking at how margins interact with each other, and how margin collapsing actually works.

The CSS Box Model

As with all articles about parts of the CSS Box Model, we should define what we mean by that, and how the model has been clarified through versions of CSS. The Box Model refers to how the various parts of a box — the content, padding, border, and margin — are laid out and interact with each other. In CSS1, the Box Model was detailed with the ASCII art diagram shown in the image below.

Depiction of the CSS Box Model in CSS1

The four margin properties for each side of the box and the marginshorthand were all defined in CSS1.

The CSS2.1 specification has an illustration to demonstrate the Box Model and also defines terms we still use to describe the various boxes. The specification describes the content box, padding box, border box, and margin box, each being defined by the edges of the content, padding, border, and margin respectively.

Depection of the CSS Box Model in CSS2

There is now a Level 3 Box Model specification as a Working Draft. This specification refers back to CSS2 for the definitions of the Box Model and margins, therefore it is the CSS2 definition we will be using for the majority of this article.

Author:Rachel Andrew
原文地址：https://www.smashingmagazine.com/2019/07/margins-in-css/