The overview page says
Q: Can I edit my game once I submit?
You can fix game breaking bugs in your submission and upload those. If your game is unpleasant or unplayable due to technical issues, we want you to fix those! You just cant add new features or gameplay. or things like that.
It's open to interpretation, but I'd call adding a webgl version a new feature rather than a game-breaking bug :(
I'll follow you, so if you add a web version after the jam ends I'll see.