Containing block horizontal constraint. A left float box's left margin edge may not be to the left of the containing block's left padding edge.
包含块的水平限制：左浮动框的margin edge不能在包含块的左padding edge框的左边。
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margin edge不能高于包含块的上padding edge。
The padding edge defines the edges of the containing block.
padding edge定义了此包含块的边缘。
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