Love it!

It is unfortunate that the scenery is positioned in such a way that lemmings end up falling by the side all the way to the bottom of the screen.

Perhaps it would look better if you removed the padding from <body> and added more pits to the scenery.

As the first paragraph says (“Drag one to carry it”) you can move them around to put them on/in the parts that you want to test their interactions with.

Other than respawning them as you scroll around, so one starts in an appropriate place as a test comes into view, and resetting them automatically, perhaps on a timer, there probably isn't a better way to properly show it off than that.

Right, I thought the same. Construct it such that a Lemming falling down from the top will have to clear every level to the bottom instead of just falling past all of them.