_checkValidNamespace = function _checkValidNamespace(element) {
      let parent = getParentNode(element);
      // In JSDOM, if we're inside shadow DOM, then parentNode
      // can be null. We just simulate parent in this case.
      if (!parent || !parent.tagName) {
        parent = {
          namespaceURI: NAMESPACE,
          tagName: 'template'
        };
      }
      const tagName = stringToLowerCase(element.tagName);
      const parentTagName = stringToLowerCase(parent.tagName);
      if (!ALLOWED_NAMESPACES[element.namespaceURI]) {
        return false;
      }
      if (element.namespaceURI === SVG_NAMESPACE) {
        // The only way to switch from HTML namespace to SVG
        // is via <svg>. If it happens via any other tag, then
        // it should be killed.
        if (parent.namespaceURI === HTML_NAMESPACE) {
          return tagName === 'svg';
        }
        // The only way to switch from MathML to SVG is via`
        // svg if parent is either <annotation-xml> or MathML
        // text integration points.
        if (parent.namespaceURI === MATHML_NAMESPACE) {
          return tagName === 'svg' && (parentTagName === 'annotation-xml' || MATHML_TEXT_INTEGRATION_POINTS[parentTagName]);
        }
        // We only allow elements that are defined in SVG
        // spec. All others are disallowed in SVG namespace.
        return Boolean(ALL_SVG_TAGS[tagName]);
      }
      if (element.namespaceURI === MATHML_NAMESPACE) {
        // The only way to switch from HTML namespace to MathML
        // is via <math>. If it happens via any other tag, then
        // it should be killed.
        if (parent.namespaceURI === HTML_NAMESPACE) {
          return tagName === 'math';
        }
        // The only way to switch from SVG to MathML is via
        // <math> and HTML integration points
        if (parent.namespaceURI === SVG_NAMESPACE) {
          return tagName === 'math' && HTML_INTEGRATION_POINTS[parentTagName];
        }
        // We only allow elements that are defined in MathML
        // spec. All others are disallowed in MathML namespace.
        return Boolean(ALL_MATHML_TAGS[tagName]);
      }
      if (element.namespaceURI === HTML_NAMESPACE) {
        // The only way to switch from SVG to HTML is via
        // HTML integration points, and from MathML to HTML
        // is via MathML text integration points
        if (parent.namespaceURI === SVG_NAMESPACE && !HTML_INTEGRATION_POINTS[parentTagName]) {
          return false;
        }
        if (parent.namespaceURI === MATHML_NAMESPACE && !MATHML_TEXT_INTEGRATION_POINTS[parentTagName]) {
          return false;
        }
        // We disallow tags that are specific for MathML
        // or SVG and should never appear in HTML namespace
        return !ALL_MATHML_TAGS[tagName] && (COMMON_SVG_AND_HTML_ELEMENTS[tagName] || !ALL_SVG_TAGS[tagName]);
      }
      // For XHTML and XML documents that support custom namespaces
      if (PARSER_MEDIA_TYPE === 'application/xhtml+xml' && ALLOWED_NAMESPACES[element.namespaceURI]) {
        return true;
      }
      // The code should never reach this place (this means
      // that the element somehow got namespace that is not
      // HTML, SVG, MathML or allowed via ALLOWED_NAMESPACES).
      // Return false just in case.
      return false;
    }
@param element a DOM element whose namespace is being checked
@returns Return false if the element has a
 namespace that a spec-compliant parser would never
 return. Return true otherwise.