The runtime is for launching games, not Steam itself. You can check the runtime selection in Compatibility tab of Steam and of each game. If your Steam Flatpak install doesn't work, the issue is likely somewhere else.
I'd suggest trying to launch the flatpak from the terminal and seeing if there's any strange logging.
This blog post reminds me of Discord's dates, often times used as release date indicators (of a game, update, stream, etc), and could be fit for
<time>for what the user intends.These could be native popovers, popovers using JS or even custom
:afterCSS pseudo elements.