Programonic

The concept

Every element is a box made of content, padding, border, and margin. With box-sizing: border-box the width includes padding and border, so the box stays the size you set. margin: 1rem auto centers a block with a set width horizontally.

Code example

CSS

.box {
  box-sizing: border-box;
  width: 20rem;
  padding: 1rem;
  border: 2px solid #333;
  margin: 1rem auto;
}

Preview example Example details

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Guided walkthroughReady

1.box {

2 box-sizing: border-box;

3 width: 20rem;

4 padding: 1rem;

5 border: 2px solid #333;

6 margin: 1rem auto;

7}

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Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
Select every element with the box class.	`—`	—
Width includes padding and border.	`—`	—
Space inside the box, around the content.	`—`	—
The auto side margins center the box.	`—`	—

Behind the scenes

How CSS processes this code

The browser matches selectors, computes styles, and paints the result. CSS does not produce console output.