Seeing some of the other comments reminded me that most of the work went into trimming the word lists so that it wasn't rude or randomly impossibly difficult.
Developing offline with the engine-only (https://github.com/playcanvas/engine) is obviously an option, but I wouldn't hold your breath for an offline editor.
They unlock a lot of advanced graphics techniques that are too slow to do CPU-side but work well in parallel on GPU. It's a pretty wide variety of things, but e.g. lighting, mesh manipulation, terrain, voxels, post-processing.
PlayCanvas[0] is the probably the closest "web-first" solution compared to Unity. Used by lots of Snap Games & Instant Games. It's written in JS which means that it's way smaller ~350kB for an empty scene if I recall.
https://conundrum.fun/
Seeing some of the other comments reminded me that most of the work went into trimming the word lists so that it wasn't rude or randomly impossibly difficult.