The inset() CSS function is used to define rectangular shapes by specifying the distances from the edges of a box: top, right, bottom, and left. It essentially acts as a shorthand for setting multiple top, right, bottom, and left values at once. This makes it particularly useful in positioning contexts, for example, with absolutely or relatively positioned elements, or when clipping elements using clip-path.

The values in inset() can be expressed using lengths (like px, em), percentages (%), or the keyword auto. If fewer than four values are provided, the function applies the same rules as the shorthand for margin or padding: one value applies to all sides, two values apply to top/bottom and left/right, and three values apply to top, left/right, and bottom.

An optional fourth parameter, known as the “round” value, allows you to add border-radius-like rounding to the corners of the defined inset rectangle. This can be useful for creating smooth clipped shapes or decorative effects.

For example, using inset() with div elements can simplify complex positioning:

div.box {
  position: absolute;
  inset: 10px 20px 30px 40px; /* top, right, bottom, left */
  background-color: lightblue;
}

You can also combine it with clip-path to create interesting masking effects:

div.clip {
  clip-path: inset(10% 5% 10% 5% round 15px);
}

Here, the element is clipped 10% from the top and bottom, 5% from the sides, and corners are rounded by 15px.

Overall, inset() provides a concise, readable, and flexible way to define rectangular offsets, replacing the need for multiple individual property declarations.