It's definitely not for lack of wanting to add a a way to delete individual saves. It's just that it is, at the very least, almost impossible. In short, with the way the engine's creator(s) built it, it would be a lot simpler for me to rebuild the entire game in a new engine than it would be to hack in this one feature to the existing game's code.
My apologies, I forgot you were using the HTML version! In that case, the local storage folder location depends on the browser that you are using to play the game.
If you are using Chrome, I believe it should be in :
~/Library/Application Support/Google/Chrome/$PROFILE/Local Storage
If Safari, that should be:
~/Library/Safari/LocalStorage