It is possible to have a web version of the demo of a game you’ve made, with the full version being available as a downloadable file (possibly for a price).
I haven’t tried, but I assume WebAssembly is supported (it’s worth giving it a try).
The only thing I’m not sure about, is if the game will show up properly on searches. If the game is not available as a full version for the web, I’m not sure if having a web demo version will classify your game as “available for the web”, and show up in relevant searches.