The isolation property determines whether an element creates a new compositing (stacking) context that isolates its descendants from blending and compositing effects of elements outside that context. In practice this means an isolated element and its children are rendered together as a single group before being blended with siblings and the background. That grouping prevents visual interactions such as blend modes from leaking in or out of the group, so the element’s internal painting is treated as self-contained.

A common interaction is with blending: using mix-blend-mode on descendants can cause them to blend with underlying content; creating an isolation context confines that blending to the group. Isolation also affects how compositing-order-dependent properties behave, because forming a new stacking context changes how elements are layered. This is related to stacking rules you might manage with z-index and positioning via position, since new contexts alter which elements participate in a given stacking context and therefore which z-index comparisons are relevant.

It’s useful to know that other CSS features can implicitly create stacking or compositing contexts — for example, opacity values and certain transform usages produce similar isolation effects for their subtree. Choosing to explicitly isolate an element gives you predictable containment of blending and compositing, which helps when you need to prevent unwanted interactions (for instance, when applying a global blend effect to a single component without affecting the rest of the page).

From a practical standpoint, use isolation to enforce visual boundaries: it’s valuable when layering complex UI components, ensuring shadows, glows, or blend effects don’t unexpectedly combine with surrounding content. Keep in mind that creating additional compositing groups can have rendering and performance implications on some devices, so apply isolation where the visual correctness it provides outweighs any cost.