[css-2015] Make proprietary extensions clause more generic (open vs. closed systems, rather than Web vs. non-Web), as requested in WG feedback.

Author: fantasai

css-2015/Overview.bs

@@ -380,9 +380,7 @@ Requirements for Responsible Implementation of CSS</h2>
 
 	To avoid clashes with future stable CSS features,
 	the CSSWG recommends the following best practices for the implementation of
-	<a>unstable</a> features and <a>proprietary extensions</a> to CSS.
-	A CSS feature is a <dfn>proprietary extension</dfn> if it is not intended for
-	or released for use on the World Wide Web.
+	<a>unstable</a> features and <a>proprietary extensions</a> to CSS:
 
 	<ol>
 		<li>
@@ -464,20 +462,33 @@ Requirements for Responsible Implementation of CSS</h2>
 			to complete standardization of such features.
 
 		<li>
-			When shipping in a production build a feature that is only meant to be used in non-Web environment,
-			it should be supported only through a prefixed syntax
-			and not exposed to the Web.
-			(Even if a feature is intended to eventually be used in the Web,
-			if it hasn't yet been standardized
-			it should still not be exposed to the Web.)
+			A CSS feature is a <dfn>proprietary extension</dfn> if it is meant for use
+			in a closed environment accessible only to a single vendor's user agent(s).
+			A UA should support such <a>proprietary extensions</a>
+			only through a vendor-prefixed syntax
+			and not expose them to open (multi-UA) environments such as the World Wide Web.
 
 			<details class=why>
 				<summary>Why?</summary>
-				The prefixing requirement allows shipping specialized features in non-Web environments
+				The prefixing requirement allows shipping specialized features in closed environments
 				without conflicting with future additions to standard CSS.
-				The restriction on exposure to the Web environment is to prevent
-				accidentally causing the public Web to depend on an unstandardized proprietary feature.
+				The restriction on exposure to open systems is to prevent
+				accidentally causing the public CSS environment
+				to depend on an unstandardized <a>proprietary extensions</a>.
 			</details>
+
+			<div class="example">
+				For example,
+				Win8 Metro apps, Firefox's XUL-based UI, and the iTunes UI
+				use extensions to CSS implemented by their respective UAs.
+				So long as these UAs do not allow Web content to access these features,
+				they do not provide an opportunity for such content
+				to become dependent on their <a>proprietary extensions</a>.
+			</div>
+
+			Even if a feature is intended to eventually be used in the Web,
+			if it hasn't yet been standardized
+			it should still not be exposed to the Web.
 	</ol>