What is the function of high, low and optimum attribute in <meter> tag?

Question

I put it in code but nothing shown...
https://code.sololearn.com/W1I52TRZ7paX/?ref=app

Answer 1

Because it changes the color of the progress bar according to that.

Answer 2

Set the value above 90 or below 20 and you'll see the difference.

Answer 3

Em... After I try, the optimum attribute is useless... isn't? Because the server set the color of bar to be green by default.