a11y: (Frontend) Burger menu links are tabbable even when menu is closed

[bryophyta]

On Frontend-rendered pages, if you're using the keyboard to navigate the site on narrower dimensions where the burger menu is present, you are able to focus the links within the menu even when it's closed.¹ This means that if you are just using this method to navigate the page, you need to tab through a whole set of invisible links to get to other elements on the page. This is not ideal because a) going through the menu links should be optional, and b) no visual feedback is given to sighted users to tell them where the links are.

Footnotes

1. From our recent DAC consultation, it sounds like there are also other methods which don't use a keyboard, but which create a similar navigation pattern, e.g. via screen gestures on mobile. ↩