IIRC, they said that you are allowed to add new platforms for your game. So adding a new build for WebGL should be ok.
What you are not allowed to do is to add new features, gameplay elements, or other enhancements.
Obviously, I'm not one of the organizers, but I do remember hearing them say this on the streams, when discussing submissions and answering questions that people were posting in the live chat. You might also be able to verify this on Discord. One of the organizers "Zesix" is usually on Discord and can probably give you a definitive answer.