Looks like this is a re-implementation in Javascript.
Alternatively, here's the original KC85/3 version in an emulator:
https://floooh.github.io/tiny8bit/kc853.html?mod=m022&file=k...
...to start a game round, you first need to press Enter before the arrow keys can be used to move the little chap around.
There were also other Boulderdash clones for the KC85, which were a bit closer to the original, like: https://floooh.github.io/tiny8bit/kc853.html?mod=m022&file=k...
That scrolling effects in the diamonds is actually a very clever hack using the KC85/3 color blinking feature where the blink frequency was controlled by a CTC counter. When I finally figured out how it works I was so excited that I wrote a whole blog post about it: https://floooh.github.io/2017/01/14/yakc-diamond-scroll.html