diff --git a/doc/webp-container-spec.txt b/doc/webp-container-spec.txt
index 2480b115..02fa893d 100644
--- a/doc/webp-container-spec.txt
+++ b/doc/webp-container-spec.txt
@@ -722,7 +722,7 @@ A file MAY contain unknown chunks:
     and [Animation](#animation) sections.
 
 Readers SHOULD ignore these chunks. Writers SHOULD preserve them in their
-original order.
+original order (unless they specifically intend to modify these chunks).
 
 ### Assembling the Canvas from fragments/frames