There are a number of different areas where core, or contributed projects, wish to implement modal dialogs. There are currently a number of concerns with making modal dialogs accessible to all users, and this has held back their implementation in Drupal Core.
I am opening this issue so that we can come up with, and implement, a plan to research, and implement, an accessible and reusable modal dialog for Drupal 8 Core.
1. Generalize the use-case for modals in core
2. Prepare a requirements document
3. Research current solutions (ideally jQuery based)
4. Select candidate solutions and test for compliance with requirements document
5. Prepare a recommendation for a candidate solution(s) to include in Drupal 8 Core