box-sizing

Over the generations, people realized that math is not fun, so a new CSS property called box-sizing was created. When you set box-sizing: border-box; on an element, the padding and border of that element no longer increase its width. Here is the same example as the previous page, but with box-sizing: border-box; on both elements:

.simple {
   500px;
  margin: 20px auto;
  -webkit-box-sizing: border-box;
     -moz-box-sizing: border-box;
          box-sizing: border-box;
}

.fancy {
   500px;
  margin: 20px auto;
  padding: 50px;
  border: solid blue 10px;
  -webkit-box-sizing: border-box;
     -moz-box-sizing: border-box;
          box-sizing: border-box;
}