The correct page is this one: https://codepen.io/adrianroworth/pen/OpeyZq
There is no full code for the arrow key controls. That is something that has yet to be created for that CSS-based crossword puzzle.
I made a start by demonstrating with the simplest example, that right-arrow control. The code for other keys is a bit more complex, as navigating between elements is not a trivial task, and is something that I have left for you to attempt for yourself.