feat(font-family-system-ui): add to playground example

JLHwung · JLHwung · commit cf5a472daf12 · 2017-02-03T14:12:30.000+08:00
diff --git a/docs/content/playground.html b/docs/content/playground.html
@@ -85,6 +85,11 @@
 /* attribute case insensitive */
 [frame=hsides i] {
 border-style: solid none;
+}
+
+/* system-ui font-family */
+body {
+font-family: system-ui;
 }
 
     </textarea>
