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

.fancy {
  width: 500px;
  margin: 20px auto;
  padding: 50px;
  border: solid blue 10px;
  -webkit-box-sizing: border-box;
     -moz-box-sizing: border-box;
          box-sizing: border-box;
}
<div class="simple">

We're the same size now!

</div>
<div class="fancy">

Hooray!

</div>

Since this is so much better, some authors want all elements on all their pages to always work this way. Such authors put the following CSS on their pages:

* {
  -webkit-box-sizing: border-box;
     -moz-box-sizing: border-box;
          box-sizing: border-box;
}

This ensures that all elements are always sized in this more intuitive way.

Since box-sizing is pretty new, you should use the -webkit- and -moz- prefixes for now, as I have in these examples. This technique enables experimental features in specific browsers. Also, keep in mind that this one is IE8+.

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