content

It is documented as part of the CSS Generated Content specification.

Syntax

/* Usage Example */
.external-link::after {
  content: '; ↗';
  color: #888;
  /* adds a small arrow after every link with this class */
}

/* Syntax */
content: normal;
content: none;
content: attr;

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