Difference between outline and border

From: http://webdesign.about.com/od/advancedcss/a/outline_style.htm

The CSS outline property is a confusing property. When you first learn about it, it's hard to understand how it is even remotely different from the border property. The W3C explains it as having the following differences:

1.Outlines do not take up space.

2.Outlines may be non-rectangular.


In addition to some other answers... here are a few more differences I can think of:

1) Rounded corners

border supports rounded corners with the border-radius property. outline doesn't. 

div {
  width: 150px;
  height: 150px;
  margin: 20px;
  display: inline-block;
  position: relative;
}

.border {
  border-radius: 75px;
  border: 2px solid green;
}

.outline {
  outline: 2px solid red;
  border-radius: 75px;
  -moz-outline-radius: 75px;
  outline-radius: 75px;
}

.border:after {
content: "border supports rounded corners";
position: absolute;
bottom: 0;
transform: translateY(100%);

}
.outline:after {
content: "outline doesn't support rounded corners";
position: absolute;
bottom: 0;
transform: translateY(100%);
}
<div class="border"></div>

<div class="outline"></div>

FIDDLE

(NB: Although firefox has the -moz-outline-radius property which allows rounded corners on outline... this property it is not defined in any CSS standard, and is not supported by other browsers (source))

2) Styling one side only

border has properties to style each side with border-top:, border-left: etc.

outline can't do this. There's no outline-top: etc. It's all or nothing. (see this SO post)

3) offset

outline supports offset with the property outline-offset. border doesn't.

.outline {
  margin: 100px;
  width: 150px;
  height: 150px;
  outline-offset: 20px;
  outline: 2px solid red;
  border: 2px solid green;
  background: pink;
}
<div class="outline"></div>

FIDDLE

Note: All major browsers support outline-offset except Internet Explorer


Further to other answers, outlines are usually used for debugging. Opera has some nice user CSS styles that use the outline property to show you where all the elements are in a document.

If you're trying to find out why an element isn't appearing where you expected or at the size you expected, add a few outlines and see where the elements are.

As already mentioned, outlines do not take up space. When you add a border, the element's total width/height in the document increases, but that doesn't happen with outline. Also you can't set outlines on specific sides like borders; it's all or nothing.

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