[css-2022] Clarify that the user agent operates on behalf of the user.

Author: fantasai

css-2022/Overview.bs

@@ -90,7 +90,8 @@ What is CSS?</h3>
 		<dt><dfn lt="user agent | UA">User Agent (UA)</dfn>
 		<dd>
 			A user agent is any program that interprets a document
-			and its associated [=style sheets=].
+			and its associated [=style sheets=]
+			on behalf of a [=user=].
 			A [=user agent=] may display a document, read it aloud,
 			cause it to be printed, convert it to another format, etc.
 			For the purposes of the CSS specifications,