Properly retrieve document body

src/services/MimeVerificationService/MimeVerificationService.ts

@@ -185,7 +185,7 @@ const normalizeVendorHtml = (htmlString: string, vendor: string): string => {
     document = window.document;
   }
 
-  const mimeBody = document.firstChild.lastChild;
+  const mimeBody = document.body;
 
   const amendNodesFunction = vendorAmendingFunctions[vendor];
   if (amendNodesFunction) {