CSS 的 place-content 屬性是一個可以用來簡化 align-content 跟 justify-content 寫法的屬性，所以這裡來示範一下當我們要用 flex 來排列內容時，同時設定了 align-content 跟 justify-content 會像這樣：
.container{
height: 200px;
background: lightblue;
display: flex;
flex-wrap: wrap;
align-items: center;
justify-content: center;
width: 300px;
}
.box{
width: 100px;
height: 50px;
background: red;
}
利用 place-content 把 align-content 跟 justify-content 合在一起，就可以寫成 place-content: center center; 其中第一個值代表的是 align-content，第二個值代表的是 justify-content。
可以設定的值包含以下這些：
start-
The items are packed flush to each other toward the start edge of the alignment container in the appropriate axis.
end-
The items are packed flush to each other toward the end edge of the alignment container in the appropriate axis.
flex-start-
The items are packed flush to each other toward the edge of the alignment container depending on the flex container's main-start or cross-start side. This only applies to flex layout items. For items that are not children of a flex container, this value is treated like
start. flex-end-
The items are packed flush to each other toward the edge of the alignment container depending on the flex container's main-end or cross-end side. This only applies to flex layout items. For items that are not children of a flex container, this value is treated like
end. center-
The items are packed flush to each other toward the center of the alignment container.
left-
The items are packed flush to each other toward the left edge of the alignment container. If the property's axis is not parallel with the inline axis, this value behaves like
start. right-
The items are packed flush to each other toward the right edge of the alignment container in the appropriate axis. If the property's axis is not parallel with the inline axis, this value behaves like
start. space-between-
The items are evenly distributed within the alignment container. The spacing between each pair of adjacent items is the same. The first item is flush with the main-start edge, and the last item is flush with the main-end edge.
baseline,first baseline,last baseline-
Specifies participation in first- or last-baseline alignment: aligns the alignment baseline of the box's first or last baseline set with the corresponding baseline in the shared first or last baseline set of all the boxes in its baseline-sharing group. The fallback alignment for
first baselineisstart, the one forlast baselineisend. space-around-
The items are evenly distributed within the alignment container. The spacing between each pair of adjacent items is the same. The empty space before the first and after the last item equals half of the space between each pair of adjacent items.
space-evenly-
The items are evenly distributed within the alignment container. The spacing between each pair of adjacent items, the main-start edge and the first item, and the main-end edge and the last item, are all exactly the same.
stretch-
If the combined size of the items is less than the size of the alignment container, any
auto-sized items have their size increased equally (not proportionally), while still respecting the constraints imposed bymax-height/max-width(or equivalent functionality), so that the combined size exactly fills the alignment container safe-
Used alongside an alignment keyword. If the chosen keyword means that the item overflows the alignment container causing data loss, the item is instead aligned as if the alignment mode were
start. unsafe-
Used alongside an alignment keyword. Regardless of the relative sizes of the item and alignment container, and regardless of whether overflow which causes data loss might happen, the given alignment value is honored.
