Placing border inside of div and not on its edge

I have a <div> element and I want to put a border on it. I know I can write style="border: 1px solid black", but this adds 2px to either side of the div, which is not what I want.

I would rather have this border be -1px from the edge of the div. The div itself is 100px x 100px, and if I add a border, then I have to do some mathematics to make the border appear.

Is there any way that I can make the border appear, and ensure the box will still be 100px (including the border)?

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.

Probably it is belated answer, but I want to share with my findings. I found 2 new approaches to this problem that I have not found here in the answers:

Inner border through box-shadow css property

Yes, box-shadow is used to add box-shadows to the elements. But you can specify inset shadow, that would look like a inner border rather like a shadow. You just need to set horizontal and vertical shadows to 0px, and the "spread" property of the box-shadow to the width of the border you want to have. So for the 'inner' border of 10px you would write the following:

div{
    width:100px;
    height:100px;
    background-color:yellow;
    box-shadow:0px 0px 0px 10px black inset;
    margin-bottom:20px;
}

Here is jsFiddle example that illustrates the difference between box-shadow border and 'normal' border. This way your border and the box width are of total 100px including the border.

More about box-shadow:here

Border through outline css property

Here is another approach, but this way the border would be outside of the box. Here is an example. As follows from the example, you can use css outline property, to set the border that does not affect the width and height of the element. This way, the border width is not added to the width of an element.

div{
   width:100px;
   height:100px;
   background-color:yellow;
   outline:10px solid black;
}

More about outline: here

Yahoo! This is really possible. I found it.

For Bottom Border:

div {box-shadow: 0px -3px 0px red inset; }

For Top Border:

div {box-shadow: 0px 3px 0px red inset; }

Although this question has already been adequately answered with solutions using the box-shadow and outline properties, I would like to slightly expand on this for all those who have landed here (like myself) searching for a solution for an inner border with an offset

So let's say you have a black 100px x 100px div and you need to inset it with a white border - which has an inner offset of 5px (say) - this can still be done with the above properties.

box-shadow

The trick here is to know that multiple box-shadows are allowed, where the first shadow is on top and subsequent shadows have lower z-ordering.

With that knowledge, the box-shadow declaration will be:

box-shadow: inset 0 0 0 5px black, inset 0 0 0 10px white;

div {
  width: 100px;
  height: 100px;
  background: black;
  box-shadow: inset 0 0 0 5px black, inset 0 0 0 10px white; 
}

<div></div>

Basically, what that declaration is saying is: render the last (10px white) shadow first, then render the previous 5px black shadow above it.

outline with outline-offset

For the same effect as above the outline declarations would be:

outline: 5px solid white;
outline-offset: -10px;

div {
  width: 100px;
  height: 100px;
  background: black;
  outline: 5px solid white;
  outline-offset: -10px;
}

<div></div>

NB: outline-offset isn't supported by IE if that's important to you.

Codepen demo