Actually, much of the time it comes down to what bugs people report. In a closely related area, IE and Edge both added spread distance rather than subtracting it on inner box-shadows until I reported it last year, then it was fixed in IE within a month: https://developer.microsoft.com/en-us/microsoft-edge/platfor....

In this particular case, they may have got the best of it because IE didn’t support outline-offset at all, and so when they implemented it they read the spec carefully.

Edge is a continuation of IE. Large swathes of the renderer have been rewritten, but when you do find bugs in Edge they’re commonly shared with IE.