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From https://github.com/servo/servo/pull/10458:

https://drafts.csswg.org/css2/visuren.html#float-rules

    A floating box's outer top may not be higher than the top of its containing block. When the float occurs between two collapsing margins, the float is positioned as if it had an otherwise empty anonymous block parent taking part in the flow. The position of such a parent is defined by the rules in the