forced-colors mode is a Windows only accessibility feature, Firefox does allow you to force enable it on other platforms, but for by default it's also mostly for Windows (it's enabled with Windows High contrast mode). For reference, it forces colors on a webpage to be contrasting, by changing them without the webpage's consent. Since iOS/macOS has no such thing, so it's reasonable to implement a placeholder where: (forced-colors) or (forced-colors: active) return false and (forced-colors: none) returns true. That will allow webpages to use it (since Firefox & Chromium support it), and expect correct results on WebKit.
WPT: https://wpt.fyi/results/css/mediaqueries/forced-colors.html
<rdar://problem/77715265>
Pull request: https://github.com/WebKit/WebKit/pull/3148
Committed 253290@main (46513cc172ce): <https://commits.webkit.org/253290@main> Reviewed commits have been landed. Closing PR #3148 and removing active labels.