Final answer:
In CSS, setting the flex-direction property to column arranges flex items from top to bottom, which switches the roles of justify-content and align-items. Justify-content now controls the vertical arrangement, and align-items handles the horizontal alignment of items within a flex container.
Step-by-step explanation:
The property flex-direction: column; in CSS defines the direction of the main axis inside a flex container, arranging the child elements (flex items) from top to bottom, instead of the default row (left to right). This impacts how justify-content and align-items properties function, as their roles are essentially swapped. The justify-content property, which normally controls the alignment along the main axis (horizontal if flex-direction is row), will now control the vertical spacing between items. On the other hand, align-items, which usually adjusts the cross-axis alignment (vertical if flex-direction is row), will govern the horizontal alignment in a flex-direction: column layout. The statement that you need to specify the container's height to control justify-content is not entirely accurate, as justify-content will still work without a set height, but setting a height can give you more control over the spacing when items don't naturally fill the container.