Earlier quoted context omitted.
Just so other readers don't mistake this for what it is: this is a pretty common technique in JS sizecoding and it's by no means Notch's invention (and he doesn't claim that it is).
When i read that i found it amazing and very clever, i honestly never thought if it was or not Notch's invention. I am not really into javascript/frontend development, but following js13k on twitter showed me a lot of things you can do to gain KBs on the client side. However, is this that common? Thanks for the info btw.
In 1k sizecoding it's rather uncommon because usually the decompression code (i.e. load the png, read the pixels) is bigger than the savings. I never dove into 13k sizecoding but I bet the payoff is much better there. I wouldn't be surprised if all decent 13k sizecoding entries would do something like this. some compression for sure, and why not PNG then?