Why did Bootstrap 3 Introduce Box-Sizing: Border-Box for All Elements?

In the transition from Bootstrap 2.3.2 to 3.0.0, a notable change was the adoption of border-box as the default box-sizing for all elements. This fundamental shift warrants exploration to understand why Bootstrap made this decision.

The official release notes indicate that the change aimed to "enhance the grid system and simplify sizing options." By encompassing all elements, the border-box attribute ensures easier calculations for sizing, particularly within the grid system.

The Bootstrap grid system is entirely fluid, with columns defined as percentages of the total width. However, the gutters maintain a fixed pixel width with 15px padding on each side. Traditionally, this combination could lead to complex calculations due to the different units employed.

However, border-box simplifies this process by including the border and padding within the declared element width. This effectively expands the area occupied by the element, making it easier to align columns and determine the overall grid layout.

Furthermore, the adoption of border-box aligns with industry best practices. Experts such as Paul Irish have advocated for its widespread adoption, citing its benefits for accurate layout control, responsiveness, and overall CSS maintainability.