diff --git a/matroska_schema_section_header.md b/matroska_schema_section_header.md
index 3e820f3aa..b117ce8e0 100644
--- a/matroska_schema_section_header.md
+++ b/matroska_schema_section_header.md
@@ -5,7 +5,7 @@ In addition to the EBML Schema definition provided by the EBML Specification, Ma
 
 | attribute name | required | definition |
 |:---------------|:---------|:-----------|
-| webm           | No       | A boolean to express if the Matroska Element is also supported within version 2 of the `webm` specification. Please consider the [webm specification](http://www.webmproject.org/docs/container/) as the authoritative on `webm`. |
+| webm           | No       | A boolean to express if the Matroska Element is also supported within version 2 of the `webm` specification. Please consider the [webm specification](http://www.webmproject.org/docs/container/) as the authoritative on `webm`. Elements with webm not set MUST NOT be found in WebM file. Ont the other end readers MAY assume these elements have their default value if they have one. |
 
 # Matroska Schema