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.
Not part of any specification.
Microsoft has a description on MSDN.
- The pseudo-elements used by IE/Edge to style other parts of a range input:
- Similar pseudo-elements used by other browsers:
- CSS-Tricks: Styling Cross-Browser Compatible Range Inputs with CSS
- QuirksMode: Styling and scripting sliders