I know it wasn’t part of the initial Flexbox spec, but it’s been widely supported in web browsers for 5 ish years now, designers use it all the time, and it’s needed a lot more often than the (usually ugly) space-around. But I can’t see it in the UIToolkit Justify class, even the 2022 version - Unity - Scripting API: Justify
Is there a particular reason for omitting this part of CSS?