A browser extension to show a hidden element to people with the extension installed

I’ve got this idea i want to hide the Agaric-insider-specific links in our public documentation by default, but display it to us lucky few in the know (or anyone who views source).

<aside class="they-live--agaric">
  Agaric's secret browser extension to easily see these notes is documented in our internal wiki.

(TODO check if this guide is up-to-date for Quantum) https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Your_second_WebExtension