How Does Bootstrap 3's `border-box` Box-Sizing Impact Layout and Grid System Calculations?

Understanding the Bootstrap 3 Shift to Box-Sizing: Border-Box

As you migrate your Bootstrap themes from version 2.3.2 to 3.0.0, you may encounter noticeable differences in dimension calculations. This is primarily due to the introduction of the following styles in bootstrap.css:

*,
*:before,
*:after {
  -webkit-box-sizing: border-box;
     -moz-box-sizing: border-box;
          box-sizing: border-box;
}

The Reasoning Behind the Change

Bootstrap 3 adopts the "border-box" value for box-sizing for all elements by default. This approach provides several advantages, particularly for the new percent-based grid system.

With "border-box," the browser calculates the final rendered box based on the declared width. Any border and padding values are included within the box, making calculations straightforward and consistent. This eliminates the potential for complex width calculations when combining fluid grids with pixels-based gutters.

Benefits for the Grid System

The shift to "border-box" has significant benefits for the Bootstrap grid system:

• Easier Sizing: The declared width of a column becomes the actual rendered width, regardless of borders or padding.
• Enhanced Consistency: The width calculation remains consistent for all columns, simplifying layout and alignment.

Additional Advantages

In addition to the grid system benefits, "border-box" box-sizing offers general improvements:

• Consistent Margins and Padding: Margins and padding values are independent of the element's content, resulting in more predictable layouts.
• Improved Cross-Browser Compatibility: The "border-box" value ensures consistent behavior across different browsers, reducing potential inconsistencies.

Conclusion

The adoption of "border-box" box-sizing in Bootstrap 3 provides a number of advantages, particularly for the new grid system. This change simplifies calculations, enhances consistency, and improves cross-browser compatibility.

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