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 ::-ms-track CSS pseudo-element represents the "track" (the groove in which the indicator slides) of an <input> of type="range".
Not part of any specification.
Microsoft has a description on MSDN.
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https://developer.mozilla.org/en-US/docs/Web/CSS/::-ms-track