Box-sizing is the method of specifying whether or not an element’s borders and padding should be included in size units.

Browser Support

Box-sizing is already supported by all major browsers so it should be really easy to get it working cross-browser.

Code Example

You can use Box-sizing like this:

div.split {
  box-sizing:border-box;
  width:50%;
  border:1em silver ridge;
  float:left;
}

More Information

More information can be found here.