ZZT Ultra is an older project to try and re-implement ZZT in ActionScript 3; it did receive an HTML5 port attempt a few years later, though.
The best way to play ZZT on the web currently is to use the dedicated Zeta emulator; it's integrated with the Museum of ZZT, as well as used by most ZZT-based games published on Itch.
Click on the link and get met with an "Unable to load Adobe Flash plugin" box. Not HTML5 so much I think. :)