No border on iframes.

packages/apputils/style/iframe.css

@@ -8,6 +8,12 @@
   height: 100%;
 }
 
+
+.jp-IFrame iframe {
+  border: none;
+}
+
+
 /*
 When drag events occur, `p-mod-override-cursor` is added to the body.
 Because iframes steal all cursor events, the following two rules are necessary