MozPowerManager.screenBrightness property defines the brightness of the screen's backlight, on a scale from 0 (very dim) to 1 (full brightness).
Setting this attribute modifies the screen's brightness. Values outside the range [0-1] are not allowed, and an error of type
NS_ERROR_INVALID_ARG is thrown if such a value is assigned to that property.
This attribute can be read and written even when the screen is disabled, but the backlight is off while the screen is disabled.
If you write a value of X into this attribute, the attribute may not have the same value X when you later read it. Most screens don't support as many different brightness levels as there are doubles between 0 and 1, so the value's precision is reduced before storing it.
var brightness = navigator.mozPower.screenBrightness
Return a number in the range [0,1].
Not part of any specification.