這篇將介紹 flex 屬性，並且深入探討 flex 究竟是如何計算伸縮的比例。
先來看一段 W3C 規範 的敘述：
In CSS, the adjoining margins of two or more boxes (which might or might not be siblings) can combine to form a single margin. Margins that combine this way are said to collapse, and the resulting combined margin is called a collapsed margin.
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