How did the admin add interactive HTML to a subreddit? Is this possible for any subreddit, or was this something special organized by the company? The intro post doesn't seem to delve into those details.

This was the company. It was for April 1st. There is more stuff added for this than meets the eye too, like automatic flair assignment.

Admins (with the red [a] tag beside their name) are Reddit Staffers, mods (with the [m] tag beside their name) are just random redditors that moderate a subreddit.

(* = only shows up if they choose to exercise admin/mod powers when posting)

It's actually pretty cool. Makes me wonder if they're working on abstracting it out so other subreddits can utilize tools like these as well.

yes, exactly. could be some cool applications if they extended the platform.