Slot with small circle that moves left or right - which is on and which is off?

Using the Firefox web browser I often see the above. For example on the options page of Adblock Plus, and elsewhere on the internet. The intention is to allow the user to turn some option on or off.

But nothing tells you if left is off and right is on, or if left is on and right is off.

Which one is it, and is it the same 100% of the time throughout the internet-universe?

And why do web designers assume that everyone will know which is which? It would have been courteous to see something that says "on" or "off" according to the position of the circle.

Note that the national conventions for physical switches vary from country to country: https://en.wikipedia.org/wiki/Light_switch#Orientation

Thanks.