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.
Summary
The ::-webkit-slider-runnable-track CSS pseudo-element represents the "track" (the groove in which the indicator slides) of an <input> of type="range".
Specification
Not part of any specification. This is a proprietary pseudo-element specific to WebKit/Blink.
See also
::-webkit-slider-thumb- Similar pseudo-elements used by other browsers:
- CSS-Tricks: Styling Cross-Browser Compatible Range Inputs with CSS
- QuirksMode: Styling and scripting sliders