--- a/layout/base/nsStyleSheetService.cpp
+++ b/layout/base/nsStyleSheetService.cpp
@@ -16,16 +16,17 @@
 #include "mozilla/dom/ContentParent.h"
 #include "mozilla/ipc/URIUtils.h"
 #include "nsIURI.h"
 #include "nsCOMPtr.h"
 #include "nsICategoryManager.h"
 #include "nsISupportsPrimitives.h"
 #include "nsISimpleEnumerator.h"
 #include "nsNetUtil.h"
+#include "nsIConsoleService.h"
 #include "nsIObserverService.h"
 #include "nsLayoutStatics.h"
 
 using namespace mozilla;
 
 nsStyleSheetService *nsStyleSheetService::gInstance = nullptr;
 
 nsStyleSheetService::nsStyleSheetService()
@@ -130,17 +131,37 @@ nsStyleSheetService::Init()
 
   return NS_OK;
 }
 
 NS_IMETHODIMP
 nsStyleSheetService::LoadAndRegisterSheet(nsIURI *aSheetURI,
                                           uint32_t aSheetType)
 {
-  nsresult rv = LoadAndRegisterSheetInternal(aSheetURI, aSheetType);
+  // Warn developers if their stylesheet URL has a #ref at the end.
+  // Stylesheet URIs don't benefit from having a #ref suffix -- and if the
+  // sheet is a data URI, someone might've created this #ref by accident (and
+  // truncated their data-URI stylesheet) by using an unescaped # character in
+  // a #RRGGBB color or #foo() ID-selector in their data-URI representation.
+  bool hasRef;
+  nsresult rv = aSheetURI->GetHasRef(&hasRef);
+  NS_ENSURE_SUCCESS(rv, rv);
+  if (aSheetURI && hasRef) {
+    nsCOMPtr<nsIConsoleService> consoleService =
+      do_GetService(NS_CONSOLESERVICE_CONTRACTID);
+    NS_WARNING_ASSERTION(consoleService, "Failed to get console service!");
+    if (consoleService) {
+      const char16_t* message = u"nsStyleSheetService::LoadAndRegisterSheet: "
+        u"URI contains unescaped hash character, which might be truncating "
+        u"the sheet, if it's a data URI.";
+      consoleService->LogStringMessage(message);
+    }
+  }
+
+  rv = LoadAndRegisterSheetInternal(aSheetURI, aSheetType);
   if (NS_SUCCEEDED(rv)) {
     const char* message;
     switch (aSheetType) {
       case AGENT_SHEET:
         message = "agent-sheet-added";
         break;
       case USER_SHEET:
         message = "user-sheet-added";