That's actually a very good question, thanks for asking!
As much as I'd like to encourage the use of JavaScript engines like Phaser, I'd also like to make this competition as open as possible. That's why I'll say it clearly: it doesn't have to be a JavaScript game, but it does have to be a Web game - it have to work in the browser without extra plugins.
If you're able to use a tool that will export your game to HTML5, then go for it. If this decision will lead to exported games flooding the competition and making JavaScript devs unhappy though, then we can get back to the discussion and maybe have a separate category for non-JS exporters. It's the very first edition of the jam that I would like to run yearly, so there's definitely room for improvements.