Always glad to see efforts to extract the essence of an idea (reactive declarative UI in this case) and put it in a small digestible package. And btw, why wouldn't you use something like this in production?

It's terribly slow and inefficient. Although, if your layout is mostly static and new components are rarely added/removed (or if performance does not matter to you) - why not? But hey, I warned you :)

There’s a pattern in the code that I don’t understand. I looks like all top-level variables are assigned with “const” whereas variable assignments in blocks use “let” even if they are not reassigned. What is the reason for that?

Nice way of showing how things work under the hood.

Needless to say the live example does not work in IE (11). It takes ages trying to make sense of the `script` tag and then gives up. I might compile a CommonJS file and throw it at IE just for fun.