diff --git a/spec/formatting.md b/spec/formatting.md
index a0fcf9a90e..66ba312063 100644
--- a/spec/formatting.md
+++ b/spec/formatting.md
@@ -121,6 +121,25 @@ These are divided into the following categories:
     when * {The value is not one.}
     ```
 
+  - **Unknown Markup errors** occur when an Expression includes
+    a reference to a markup handler which cannot be resolved.
+    
+    For example, attempting to format the following messages
+    MUST result in an Unknown Markup error if done within a context that
+    does not provide a handler for the markup `+foo` or `-foo`
+    
+    ```
+    {There are {+foo option=bar}no{-foo} wildebeest here}
+    ```
+
+    ```
+    {There are {+foo option=bar}no wildebeest here}
+    ```
+
+    ```
+    {There are no{-foo} wildebeest here}
+    ```
+
 - **Selection errors** occur when message selection fails.
 
   - **Selector errors** are failures in the matching of a key to a specific selector.