svg() {
      const svg = this._svg,
        bg = this._bgcolor;
      if (!svg) return null;
      let node;
      if (bg) {
        svg.removeAttribute('style');
        node = domChild(svg, RootIndex, 'rect', svgns);
        setAttributes(node, {
          width: this._width,
          height: this._height,
          fill: bg
        });
      }
      const text = serializeXML(svg);
      if (bg) {
        svg.removeChild(node);
        this._svg.style.setProperty('background-color', bg);
      }
      return text;
    }
Returns an SVG text string for the rendered content,
or null if this renderer is currently headless.