Editoria11y v1 filled an unmet need for Drupal: a robust but still intuitive accessibility checker that worked like magic.

For all its code, it was also fairly straightforward under the hood: lots of jQuery and IF statements!

For Editoria11y v2, I had a lengthy wishlist:

Learn to let go of the jQuery dependency
Let the user dismiss yellow alerts...and synchronize the dismissal with other users
Let Drupal collect findings into site-wide reports
Be able to check really complex HTML, with multiple content root containers and shadow DOM components.
Leverage Drupal translations
Add some basic automated testing so I'd stop randomly breaking stuff
The catch? Everything on that list was something I did not know how to do.

But here we are, thanks to a long list of mentors and a lot of trial and error.

So let us take a it's-new-to-some-of-us, down-with-imposter-syndrome tour of the techniques it takes to build a complex Web application out of Vanilla JS, Drupal APIs and Shadow DOM.

John Jameson
https://www.drupalcampnj.org/sessions/editoria11y-v2-building-drupal-integrated-accessibility-checker