It seems like both BFC and IFC can have absolutely positioned children.
It's a bit strange, but consider the following HTML:
<html><body>foobar<img style="position: absolute"></body></html>
In such a document, the <img> element is an absolutely positioned child
of a block-level element (<body>) with no block-level children.
An IFC is established for <body>, and needs to handle layout for <img>.
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