Optimum horizontal constraint. A left float box must be put as far to the left as possible within its containing block.
最佳水平限制：浮动框必须被置于此包含块内向左尽量远的地方。
2
Previous element vertical constraint. A left float box's top margin edge may not be higher than the top margin edge of any block or floated box generated by an earlier element in the source document.