Non-standard: This feature is not standardized. We do not recommend using non-standard features in production, as they have limited browser support, and may change or be removed. However, they can be a suitable alternative in specific cases where no standard option exists.
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.