Optimisations

Optimised CSS selectors.
Changed markup in `linenos-test` to inhibit line numbers appearing in YAML examples when testing code fences.

_sass/code.scss

@@ -83,8 +83,8 @@ figure.highlight {
   border: 0;
   box-shadow: none;
 
-  tr > td,
-  td > pre {
+  td,
+  pre {
     @include fs-2;
     min-width: 0;
     padding: 0;