Blank game screen on Level 19
Solution 1:
Unfortunately, as we've just discovered, it looks like level 19 doesn't render on any browser except Chrome.
It's a known issue and we're working hard to fix it (https://github.com/AlexNisnevich/untrusted/issues/105).
In the meantime, there is a sort of a "cheat" you can do: click within the blank game screen and keep pressing the Up key. Eventually you will move on to the next level.