The CSS box-sizing property is used to define how borders, padding, width, and height interact with each other. The default value is content-box, meaning that width and height refer to an element's content, and then padding and borders are added around it. Consider the following:

.mybox {
    border: 1px solid black;
    padding: 5px;
    width: 100px;
}

The actual width of this box is 112 pixels. That's because the 100 pixels specified by width indicates how much area the content should take up, then 5 pixels are added on each side for padding, and 1 pixel on each side for the border.

When you change box-sizing to border-box, the calculation is done differently:

.mybox {
    box-sizing: border-box;
    border: 1px solid black;
    padding: 5px;
    width: 100px;
}

This box has an actual width of 100 pixels while the space available for content is only 88 pixels (100 - 5px padding - 5px padding - 1px border - 1px border). Many consider the border-box setting to be more logical and more like how these properties should work.

There is only a problem using box-sizing when you need to support Internet Explorer 6 and 7. These browsers do not support box-sizing and so will interpret the box model properties differently.

Rule ID: box-sizing

This rule warns when the box-sizing property is used. The intent is to ensure that developers realize this property is not supported in older browsers such as Internet Explorer 6 and 7.

The following patterns are considered warnings:

.mybox {
    box-sizing: border-box;
}
.mybox {
    box-sizing: content-box;
}

• Box Sizing
• https://developer.mozilla.org/En/CSS/Box-sizing