I'm not part of the jam organizers but I'm almost certain that it would be okay to build the game as WebGL and add it to the jam page. I think the rules were mainly concerned about not developing new features or more content. This is what they say on the jam info page: "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." You could argue that because you have only Windows build your game is unplayable to some potential players with different operating system for example. And if you are not changing the game in any other way than adding another build version, I wouldn't call it implementing new content either.
Still, if you want to be sure, wait for the organizers to answer. But realistically speaking, I have it hard time imagining that the organizers would punish anyone who just want to make their game more easily accessible. They seem pretty nice people and the rules appear to be very relaxed anyway.
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.
