The border-image-outset CSS property sets the distance by which an element's border image is set out from its border box.

The parts of the border image that are rendered outside the element's border box with border-image-outset do not trigger overflow scrollbars and don't capture mouse events.

Try it

border-image-outset: 0;

border-image-outset: 15px;

border-image-outset: 30px;

border-image-outset: 40px;

<section id="default-example">
  <div id="example-element">This is a box with a border around it.</div>
</section>

#example-element {
  width: 80%;
  height: 80%;
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 50px;
  background: #fff3d4;
  color: black;
  border: 30px solid;
  border-image: url("/shared-assets/images/examples/border-diamonds.png") 30
    round;
  font-size: 1.2em;
}

Syntax

/* <length> value */
border-image-outset: 1rem;

/* <number> value */
border-image-outset: 1.5;

/* top and bottom | left and right */
border-image-outset: 1 1.2;

/* top | left and right | bottom */
border-image-outset: 30px 2 45px;

/* top | right | bottom | left */
border-image-outset: 7px 12px 14px 5px;

/* Global values */
border-image-outset: inherit;
border-image-outset: initial;
border-image-outset: revert;
border-image-outset: revert-layer;
border-image-outset: unset;

The border-image-outset property may be specified as one, two, three, or four values. Each value is a <length> or <number>. Negative values are invalid and will cause the border-image-outset declaration to be ignored.

1. If one value is specified, it applies to all four sides.
2. If two values are specified, the first applies to the top and bottom and the second to the left and right.
3. If three values are specified, the first applies to the top, the second to the left and right, and the third to the bottom.
4. If four values are specified, they apply to the top, right, bottom, and left in that order (clockwise).

Values

<length>

The size of the border-image outset as a dimension — a number with a unit.

<number>

The size of the border-image outset as a multiple of the element's corresponding border-widths. For example, if an element has border-width: 1em 2px 0 1.5rem, and border-image-outset: 2, the final border-image-outset would be calculated as 2em 4px 0 3rem.

Formal definition

Formal syntax

border-image-outset = 
  [ <length [0,∞]> | <number [0,∞]> ]{1,4}  

Examples

Outsetting a border image

HTML

<div id="outset">This element has an outset border image!</div>

CSS

#outset {
  width: 10rem;
  background: #cceeff;
  border: 1.4rem solid;
  border-image: radial-gradient(#ffff22, #5555ff) 40;
  border-image-outset: 1.5; /* 1.5 × 1.4rem = 2.1rem */
  margin: 2.1rem;
}

Result

Specifications

Browser compatibility

See also

• Backgrounds and borders
• Learn CSS: Backgrounds and borders
• Border images in CSS: A key focus area for Interop 2023 on MDN blog (2023)