[css-fonts-4] close w3c#4468 Add a ui-sans-serif keyword to go with ui-serif

Author: svgeesus

css-fonts-4/Overview.bs

@@ -427,7 +427,21 @@ Standard font families</h4>
                     <img alt="sample ui-serif font" src="images/ui-serif.png" >
                     <figcaption>Sample ui-serif font on macOS Catalina and iOS 13: New York</figcaption>
                 </figure>
-            </div>
+			</div>
+
+		<dt id="ui-sans-serif-def"><dfn>ui-sans-serif</dfn>
+			<dd>
+				This font family is used for the sans-serif variant of the system's
+				user interface. The purpose of ''ui-sans-serif'' is to allow web
+				content to integrate with the look and feel of the native OS.
+
+				<!-- TOD add sample
+					<div class="example">
+					<figure>
+						<img alt="sample ui-serif font" src="images/ui-serif.png" >
+						<figcaption>Sample ui-serif font on macOS Catalina and iOS 13: New York</figcaption>
+					</figure>
+				</div> -->
 
         <dt id="ui-monospace-def"><dfn>ui-monospace</dfn>
         <dd>
@@ -6183,7 +6197,7 @@ Changes
 
 <ul>
 	<li>Add note encouraging UAs to also implement platform-specific names for standard font families</li>
-	<li>Add ui-serif, ui-monospace, and ui-rounded generic font families</li>
+	<li>Add ui-serif, ui-sans-serif, ui-monospace, and ui-rounded generic font families</li>
 	<li>Remove font-variant @font-face descriptor per WG resolution</li>
 	<li>Remove font-min-size and font-max-size per WG resolution</li>
 	<li>Clarify that font-style 'normal' animates as 'oblique 0deg'</li>