https://playcanvas.com/ describes itself as “the web-first game engine”. It appears to be yet another attempt to build an Unreal/Unity engine with an integrated editor suite, with “entirely in your browser” as a flagship feature.
Also they were acquired by Snap a while ago. They've built a pretty impressive engine IMO. There are some pretty cool games and non-game experiences built on it. Such as https://venge.io
It's a bit more complicated than than. The front-end framework that the PlayCanvas Editor is built on (PCUI) is also open source:
https://github.com/playcanvas/pcui
Other open source tools like the glTF viewer are also built on it:
https://github.com/playcanvas/playcanvas-viewer
The rest of the Editor code is unminified and unobfuscated. The is done deliberately so devs can write extensions, report bugs and generally understand how it works. More of the Editor codebase will be open sourced over time.