fix(UI): Fix CSS bug regarding the loading animation (#445)

* This issue was spotted after adding an HTML5 DOCTYPE, given browsers
  were certainly relying on quirks-mode to workaround it up to now.

* href: fe0fe8b#commitcomment-57819951

static/css/learnocaml_main.css

@@ -210,7 +210,7 @@ body {
   }
   #learnocaml-main-loading {
     position: absolute;
-    top: 0; left: 360; right: 0; bottom: 0;
+    top: 0; left: 360px; right: 0; bottom: 0;
   }
 }
 /* Medium screen: one column w/ popup menu */