The box-shadow property is used to paint one or more shadows around an element’s frame, creating depth and separation from the page background. Unlike borders, shadows do not change layout or the element’s box dimensions; they are purely visual and are composited on top of or beneath the element’s background. You can layer multiple shadows to build subtle depth or dramatic emphasis, and shadows may be applied so they appear to emanate from outside the element or to appear contained within it, resulting in different visual effects.

Shadows interact with an element’s rounded corners and background filling: a clipped corner will produce a shadow that follows the curved edge, so combining border-radius with shadows is a common way to create soft, card-like surfaces. Because shadows are painted relative to an element’s box, they also work together with the element’s background to define perceived depth; a semi-transparent background color plus a gentle shadow can convey elevation, while a strong shadow against a light background gives a floating appearance. When elements are transformed (rotated, scaled, or translated), their shadows are transformed along with them, so consider the visual relationship between an element and its shadow when using transform.

Rendering and performance are important considerations when using shadows extensively. Complex or large shadows can increase paint and compositing work, especially when many elements or frequent animations are involved; in such cases it’s common to use a compositing hint like will-change or to render a shadow via a rasterized asset or pseudo-element to reduce repeated repaints. An alternative for some use cases is to use image-based or filtered effects - for instance the CSS drop-shadow via filter - which can sometimes be cheaper or offer different blending characteristics than native box shadows.

From a practical design standpoint, subtlety usually works best: mild, low-contrast shadows provide separation without overpowering content, while larger, higher-contrast shadows should be used sparingly for emphasis. Shadows should not be relied on as the only indicator for interactive focus or state; pair them with clear focus outlines or other cues such as color changes and borders - for focus outlines consider outline - and be mindful of stacking contexts and layering when overlapping elements (which is controlled in part by z-index). Finally, consider accessibility and color contrast: very subtle shadows may be invisible to some users or on certain displays, so test designs across backgrounds and devices.