The specification is currently lose about whether or not pseudo-elements can have the flow-into property:

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The ‘flow-into’ property does not apply to the ::first-line and ::first-letter pseudo-elements.

The effect of ‘flow-into’ on generated content such as ::marker, ::before and ::after is undefined. This may change depending on implementation feedback.
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It seems it would be better to disallow flow-into on pseudo-elements because a) it can break the model of some pseudo elements (e.g., ::before would no longer be 'before' its element's content if moved to a flow) and b) it causes issues with the object model's NamedFlow.getContentNodes() which assumes that all that can be flown into a region is a Node (and there is no Node API to pseudo-elements).

The recommendation is to strictly disallow flow-into on all pseudo-elements.