What does `* { box-sizing: border-box; }` do?

What are its advantages?

High
CSS

* { box-sizing: border-box; } makes every element on the page use the box-sizing: border-box approach for calculating the elements height and width.

What's the difference?

By default, elements have box-sizing: content-box applied, and only the content size is being accounted for if an element has height and width specified. box-sizing: border-box changes how the width and height of elements are being calculated, border and padding are also being included in the calculation. The height of an element is now calculated by the content's height + vertical padding + vertical border width. The width of an element is now calculated by the content's width + horizontal padding + horizontal border width.

The following table indicates whether the property is included in the element's calculation of height and width when it has the respective box-sizing:

Propertybox-sizing: content-box (default)box-sizing: border-box
contentYesYes
paddingNoYes
borderNoYes
marginNoNo

Advantages

Taking into account paddings and borders as part of the box model resonates better with how designers actually imagine content in grids. This is a much more intuitive way to think about boxes and hence many CSS frameworks set * { box-sizing: border-box; } globally, so that all elements use such a box model by default.

References