It looks like this commit removed it.

The parser should accept at-rules for margin boxes inside @page's body:

@page {
  color:red;
  @bottom-left {
    color:green;
  }
}

See: http://www.w3.org/TR/css3-page/#margin-at-rules

Actually, it just returns an error: Error: @page missing '}'

(Moved from https://github.com/reworkcss/css-parse/issues/80)