Styling based on element-ids, it is impossible for themes/plugins to display multiple cards on a single page. Using classes is a more flexible approach. The element-ids are maintained for backwards compatibility with existing plugins/themes.
Plugins like https://github.com/discourse/discourse-calendar add extra HTML (e.g. icons) to user/group mentions. Clicking on those extra elements used to only flash a blank card. Now, the card opens properly.
This was primarily an issue on mobile, where after clicking the "Messsage" button in the user card, the composer would load behind the user card overlay.
