Oh man, making a 7-segment display was my favorite question to ask frontend developers. Eventually turning it into a real clock if they were fast enough. Captures lots of basic CSS positioning, JS timers behavior and general ideas like breaking things into components etc
I'm guessing when you say "ask frontend developers" you mean "at job interviews". But I like to think you mean something like "at cocktail parties" or other social situations.
I've been looking for something exactly like this, actually! I'm making a hardware device that is a habit tracker, and the screen will be a few 7 segment displays. Being able to simulate the displays like this is very nice. Also very cool that it displays whatever you type in!
Edit: I now notice it mentions Spanish trains, although the link 404s
(Submitted title was "Not mine, but it's a website where you can use a segment display")