In 2019 they released a web version of minecraft classic, as a quirky marketing thing for the game's anniversary. But what they released turned out to be built on my open-source voxel engine, and when I dug around their code I realized they'd yoinked my engine a solid two years earlier.
And the demo they released was probably not two year's of work, so my theory is that somebody at Mojang investigated the idea of minecraft-but-JS, and made a demo but then decided not to pursue it, and then later on it got recycled for the marketing demo. (which, annoyingly for me, they pretended was an old alpha build of Minecraft instead of a new thing built on open source.)
The demo is still live, though the multiplayer stopped working the same day it launched: