diff --git a/doc/PROTOCOL-WEB.md b/doc/PROTOCOL-WEB.md
index 3fe6deb14f31773e7ae851692a864010590f7cbc..b6ea3ecabe788e095bbc137d3b8ddfa32b378b40 100644
--- a/doc/PROTOCOL-WEB.md
+++ b/doc/PROTOCOL-WEB.md
@@ -37,7 +37,8 @@ Content-Type
 
 1. application/grpc-web
   * e.g. application/grpc-web+[proto, json, thrift]
-  * the message format, e.g. +proto, +json, should always be specified by the sender
+  * the sender should always specifiy the message format, e.g. +proto, +json
+  * the receiver should assume the default is "+proto" when the message format is mssing in the C-T, i.e. "application/grpc-web"
 2. application/grpc-web-text
   * text-encoded streams of “application/grpc-web”
   * e.g. application/grpc-web-text+[proto, thrift]