https://drafts.csswg.org/css-box-4/#margin-trim
<rdar://problem/100336068>
+CC Sammy -- I think you shipped this already[1] and this bug can be closed right? (Is there another bug tracking this work that this should be duped against?) [1] https://webkit.org/blog/13966/webkit-features-in-safari-16-4/#css
Thanks Daniel!