createElementNS(namespaceURI, name, className) {
    const element = this._document.createElementNS(namespaceURI, name);
    if (className) {
      for (const token of className.split(/\s+/)) {
        if (token) element.classList.add(token);
      }
    }
    return element;
  }
@param {string} namespaceURI
@param {string} name
@param {string=} className
@return {Element}