-webkit-box-sizing: border-box;

This CSS property sets the sizing behavior of an element's box model.

The `border-box` value specifies that the element's total width and height should include the content, padding, and border dimensions. This means that any padding or border added to the element will be included in the element's specified width and height, rather than adding to it.

This can be useful for layout purposes, as it allows elements to be sized more predictably and consistently, without having to account for extra padding or border dimensions.

The `-webkit-` prefix is used to indicate that this property is specific to the WebKit rendering engine, which is used by browsers such as Safari and Chrome. However, the `border-box` value is also supported by other modern browsers without the need for the prefix.