Placing border inside of div and not on its edge

Set box-sizing property to border-box:

div {
    box-sizing: border-box;
    -moz-box-sizing: border-box;
    -webkit-box-sizing: border-box;
    width: 100px;
    height: 100px;
    border: 20px solid #f00;
    background: #00f;
    margin: 10px;
}

div + div {
    border: 10px solid red;
}

<div>Hello!</div>
<div>Hello!</div>

It works on IE8 & above.

You can also use box-shadow like this:

div{
    -webkit-box-shadow:inset 0px 0px 0px 10px #f00;
    -moz-box-shadow:inset 0px 0px 0px 10px #f00;
    box-shadow:inset 0px 0px 0px 10px #f00;
}

Example here: http://jsfiddle.net/nVyXS/ (hover to view border)

This works in modern browsers only. For example: No IE 8 support. See caniuse.com (box-shadow feature) for more info.