isolation

It is documented as part of the Compositing and Blending specification.

Syntax

/* Usage Example */
.widget-container {
  isolation: isolate;
  /* prevents children with blend modes from bleeding into the page background */
}

/* Syntax */
isolation: auto;
isolation: isolate;

/* Global values */
isolation: inherit;
isolation: initial;
isolation: revert;
isolation: revert-layer;
isolation: unset;