Non-standard: This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
The ::-moz-range-progress CSS pseudo-element is a Mozilla extension that represents the lower portion of the track (i.e., groove) in which the indicator slides in an <input> of type="range". This portion corresponds to values lower than the value currently selected by the thumb (i.e., virtual knob).
Note: Using ::-moz-range-progress with anything but an <input type="range"> doesn't match anything and has no effect.