diff --git a/index.bs b/index.bs
index 61a8d0f..6f75da9 100644
--- a/index.bs
+++ b/index.bs
@@ -460,8 +460,8 @@ each other. A shared rendering engine may form an [=equivalence class=], for exa
 
 To <dfn>create an arbitrary brand</dfn>, the [=user agent=] MUST run these steps:
 
-    1. Let |arbitraryBrand| be a [=/string=] composed of [=ASCII lower alpha=], [=ASCII upper alpha=], or 0x20 (SP), no
-         longer than twenty [=ASCII bytes=].
+    1. Let |arbitraryBrand| be a [=/string=] composed of [=ASCII alpha=]. |arbitraryBrand| MUST contain one or more
+         0x20 (SP) bytes and be no longer than twenty [=ASCII bytes=].
     1. Let |arbitraryBrandList| be the result of <a lt="split on ASCII whitespace">splitting |arbitraryBrand| on ASCII
          whitespace</a>.
     1. Let |greaseyStack| be a [=stack=].