As per this [Mozilla bug](https://bugzilla.mozilla.org/show_bug.cgi?id=1635675), there are already vendor-specific selectors to style buttons rendered within `<input type=file>` controls for Webkit and Trident engines. Webkit alone represents the majority market share, EdgeHTML is now "legacy", leaving Gecko/Quantum as the only current major engine without it. Mozilla is reluctant to add their own vendor-prefixed selector when the sensible choice would be to make `::file-upload-button` official.