Shoot, the unpredictable godot build situation really sucks! Best you can do I think is have a downloadable windows version AND a webgl version. That way, people come and see you put the effort in for HTML. And if they get invested in the artstyle/mechanics but have bug issues, then they might see a line in your page: "If the webgl version is buggy, it's cause of godot. Those bugs don't occur in the windows version if you'd like to download" or something like that.
As for a tutorial, I think the right way to go might involve zooming in on the normal distribution, showing the numbers 1 through 13, and then explaining that the y axis represents the probability of rolling a given number. And that since 13 has the highest probablility, the player will roll that number.