Adding warnings for non-standard escape sequence usage

src/sass/_base.scss

@@ -6,6 +6,7 @@ $light-gray: tint($gray, 25%);
 $tan: #cbcbba;
 $red: #b3151a;
 $blue: #dae9e5;
+$yellow: #f8ca00;
 $black: #000;
 $white: #fff;
 

src/sass/main.scss

@@ -383,6 +383,21 @@ header {
   }
 }
 
+#warnings {
+  @include adjust-font-size-to($base-font-size, 1);
+  font-weight: bold;
+  background-color: $yellow;
+  display: none;
+
+  li {
+    margin: rhythm(1/4);
+  }
+
+  body.has-results & {
+    display: block;
+  }
+}
+
 #regexp-render {
   background: $white;
   width: 100%;