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-fill CSS pseudo-element represents the filled-in portion of a <progress> element. This pseudo-element is non-standard and specific to Internet Explorer 10+, hence the vendor prefix.
::-ms-fill { style properties } Not part of any specification. Microsoft has a description on MSDN.
progress::-ms-fill {
background-color: orange;
} <progress value="10" max="50">
A progress bar using this style might look something like this:

| Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari |
|---|---|---|---|---|---|
| Basic support | No support | No support | 10.0 | No support | No support |
| Feature | Android | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile |
|---|---|---|---|---|---|
| Basic support | No support | No support | ? | No support | No support |
::-moz-progress-bar::-webkit-progress-bar::-webkit-progress-value::-webkit-progress-inner-element
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Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/CSS/::-ms-fill