So all the flex-* values that the FlexEnumConverters allow aren't needed here because only CSS can use them but not XAML? How do you actually prevent those values from being used in XAML?

we don't prevent it's usage at this time unfortunately, but we frown at it. What I'm realising right now is that we prevent XAML compilation of that value, due to [TypeConverter] attached.

that's the same story as the FlowDirection PR we merged a few days ago

I know, and I didn't understand how XAML was going to not support those values there either, but now I have my answer. With the new design time TypeConverters the "frown at it" will become much more visible to everyone. I guess we'll see if our users will frown at that. :-)