I'd hope most professional software engineers could do this in an afternoon or so?

Most professional software engineers have never written a game and don't do web work, so I somehow doubt that.

With TCL/TK it's a matter of less than 2 hours.

Depends on the rules. Can I look up other space invaders games on github first? Can I use a game framework?

Just JS / HTML docs I probably could not.

No preexisting framework.