Containing block vertical constraint. A left float box's top margin edge may not be higher than the containing block's top padding edge.
包含块的垂直限制：左浮动框的上marginedge不能高于包含块的上padding edge。
Containing block vertical constraint. A left float box's top margin edge may not be higher than the containing block's top padding edge.
包含块的垂直限制：左浮动框的上marginedge不能高于包含块的上padding edge。
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