We recently participated in our very first Game Jam (GMTK GameJam). We didn't know too much about how to sell our game, but I think it is pretty evident that games that are playable in browser get much more engagement that games that one has to download (at least when the game doesn't sell itself super good right on the submissions page). We only submitted a Windows build, since we ran out of time to also export to WebGL.
It would be interesting if itch.io would release some overall statistics about how much the available platform for a GameJam game influences the number of ratings it gets. Personally, I feel that it must be quite high. I've caught myself also heavily favoring playing browser games, rather than playing a downloadable game, unless the downloadable game looks super interesting.
I guess we all know that making something more available is always better than making it available to fewer people but I would be really interesting in how large this effect is!