visitDocument(node) {
    var buf = this.indent + '@' + node.vendor + 'document ' + node.document;
    buf += '\n';
    ++this.indents;
    buf += this.visitRules(node.rules);
    --this.indents;
    return buf;
  }
Visit Document `node`.`

@param {Document} node
@return {String}
@api private