Currently, the spec states that if you have a block container with both flow-from and flow-into properties set, nothing is visually formatted:

#my-block-container {
  flow-into: some-flow;
  flow-from: some-flow;
}

It would be better if we could define the properties such that this put the contents of #my-block-container into the named flow, and put the box in the region chain for that flow. The best we can do now is:

#my-block-container > * { flow-into: some-flow; }
#my-block-container { flow-from: some-flow; }

But this loses any and all direct content of #my-block-container