emscripten : fix performance issues on some browsers

For example, Safari becomes super slow when the outline of the HTML
element is enabled.

src/style.css

@@ -11,6 +11,7 @@ body {
     font: sans-serif;
 }
 .no-sel {
+    outline: none;
     -moz-user-select: none;
     -webkit-user-select: none;
     -webkit-touch-callout: none;