Currently display: compact results in display: block rendering unless the experimental region based column implementation is turned on. When that is turned on we don't follow the current version of the spec... so it's not clear why that's there.
The feature is used on 0.02% of page views, so I don't think we can remove it, but I think we should make the value of compact compute to "block" which results in identical behavior and means that our code doesn't need to know about it.
Hopefully someday we can just remove the feature entirely (and get it removed from the spec).
- E