失控的 margin-top 是指在使用 margin 時常出現的問題，內元素的 margin 被外層所吸收，導致無法正確的顯示，這我們稱為 Collapsing margins ，本篇來教大家怎麼應對這個問題。
情境
在製作 Banner 的時候，通常需要一個 Logo 在畫面的左上，於是乎我們就會準備一個 div 把美美的 logo 往裡面塞。
如圖所示，我們的 Logo 還希望離上方有些為的距離，依邏輯來說物件離邊緣要有距離，我們就會幫他補上 margin ，於是乎就會出現如下的問題。
See the Pen Collapsing margins (for blog) by Wcc723 ( @Wcc723 ) on CodePen .
仔細看看上面所發生的問題，內元素的 margin 沒有起了作用，而是被外層所吸收。
失控的 Margin top
在遇到這個問題時，如果不熟的情況下就會說：「Bug 啦！」，通常只能用各種
拋開羞恥心其它的方式去解決這個問題，但只要拿抓住發生的原因，就能輕易的去克服它。
這個問題在 w3c 有提到這問題稱為「 Collapsing margins 」，我將這部分縮減為以下幾點：
- 內外層均為 “塊狀” 元素。
- 外層塊狀元素沒有 border、padding、overflow。
- 內層第一個塊狀元素(空元素不算)，不包含 float、absolute。
所以如果要避開發生這個問題，就是避免存在以上這些問題，比如說不使用塊狀元素、外層加入padding、內層加入 float 等等，以下就列出一些解決方案。
See the Pen Collapsing margins (resolve for blog) by Wcc723 ( @Wcc723 ) on CodePen .
這也是我們在與學員之間的問答中常遇到的，這次找個機會寫個文章跟大家分享分享。
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