Important: When you set the width and height properties of an element with CSS, you just set the width and height of the content area. To calculate the full size of an element, you must also add padding, borders and margins.

Use the box-sizing property to keep the width at 300px, no matter the amount of padding

若div設定width 和 height,只設定了文字的面積,總物件大小需再加上padding, borders 和 margins。

若要固定物件大小,需設 box-sizing: border-box; 如此能無視padding的大小。

Margin Collapse

Top and bottom margins of elements are sometimes collapsed into a single margin that is equal to the largest of the two margins.

This does not happen on left and right margins! Only top and bottom margins!



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