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。

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If the box does not have a baseline, align the bottom margin edge with the parent's baseline.

如果该行内框没有基线，将框的底线对齐父框的基线。

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If the box does not have a baseline, align the bottom margin edge with the parent's baseline.

如果该行内框没有基线，将框的底线对齐父框的基线。

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