Im using godot v3.3
Hmmmm...I've opened the file in Firefox and I get a network error. From what I've seen on google and my general knowlege of Godot 3, I assume it's a bug with v3.3 because I first used 3.5 and HTML worked fine. Try exporting in 3.5 or 3.6 (the godot file should still work with later versions of godot 3) and see if that changes anything.