整个页面的切换是依赖滑条的
Displaying items in a single dimension[维度] as a row[水平] or as a column[竖直].
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flex: property sets how a flex item will grow or shrink to fit the space. It is a shorthand for flex-grow, flex-shrink, and flex-basis.
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align-: the cross-axis(x).
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justify-: the main-axis(y).
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align-items: alignment of items on the Cross Axis
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justify-content：distributes space between and around content items along the main-axis.
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align-content: distributes space between and around content items along the cross-axis.
(flex中多子元素才生效)
叫cross和main（交叉和主轴） 是因为flex-direction可以指定主轴的走向（垂直或竖直）
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order: Sequential[先后顺序]
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flex-grow: 占比放大
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flex-shrink: 占比缩小
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flex-basis: 占固定空间
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flex: property[属性] sets how a flex item will grow or shrink to fit the space. It is a shorthand for flex-grow, flex-shrink, and flex-basis.(建议优先写这个)
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align-self: 属性允许单个项目有与其他项目不一样的对齐方式，可覆盖align-items属性。 默认值为auto，表示继承父元素的align-items属性，如果没有父元素，则等同于stretch。
