docs: public Document clean(Document dirtyDocument) {
          Validate.notNull(dirtyDocument);
  
          Document clean = Document.createShell(dirtyDocument.baseUri());
          if (dirtyDocument.body() != null) // frameset documents won't have a body. the clean doc will have empty body.
              copySafeNodes(dirtyDocument.body(), clean.body());
  
          return clean;
      }