shape-margin

It is documented as part of the CSS Shapes specification.

Syntax

/* Usage Example */
.circle-wrap {
  float: left;
  shape-outside: circle(50%);
  /* Pushes the text 20px away from the curve of the circle */
  shape-margin: 20px;
}

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