Add tabindex=0 to all code blocks

Note: this is a bit hacky. Ideally, I'd like to do this at the
markdown level, since this doesn't serve folks who have
JS disabled (among many other things).

Also, curious to see how this interacts with custom highlighters;
needs some more testing!

assets/js/just-the-docs.js

@@ -566,8 +566,17 @@ jtd.onReady(function(){
   {%- endif %}
 });
 
-// Copy button on code
+// Accessibility: set tabindex=0 on each code highlight block, so screenreaders
+// can focus over (particularly important if there's horizontal scroll)
+// see: https://dequeuniversity.com/rules/axe/4.9/scrollable-region-focusable?application=axeAPI
 
+jtd.onReady(() => {
+  document
+    .querySelectorAll("div.highlight")
+    .forEach(codeBlock => codeBlock.setAttribute("tabindex", "0"));
+});
+
+// Copy button on code
 
 {%- if site.enable_copy_code_button != false %}