diff --git a/doc/PROTOCOL-WEB.md b/doc/PROTOCOL-WEB.md
index 5f37df9b0f3..10998899c43 100644
--- a/doc/PROTOCOL-WEB.md
+++ b/doc/PROTOCOL-WEB.md
@@ -37,6 +37,8 @@ Content-Type
 
 1. application/grpc-web
   * e.g. application/grpc-web+[proto, json, thrift]
+  * the sender should always specify the message format, e.g. +proto, +json
+  * the receiver should assume the default is "+proto" when the message format is missing in Content-Type (as "application/grpc-web")
 2. application/grpc-web-text
   * text-encoded streams of “application/grpc-web”
   * e.g. application/grpc-web-text+[proto, thrift]