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 ::-ms-check CSS pseudo-element represents the checkmark of an <input> of type="checkbox" or type="radio".
Specification
Not part of any specification.
Microsoft has a description on MSDN.