The alignment is done after the lengths and auto margins are applied, meaning that, if in a Flexbox layout there is at least one flexible element, with flex-grow different from 0, it will have no effect as there won't be any available space.

start

The items are packed flush to each other toward the start edge of the alignment container in the main axis.

end

The items are packed flush to each other toward the end edge of the alignment container in the main 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 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 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 along the main axis.

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.

normal

The items are packed in their default position as if no justify-content value was set. This value behaves as stretch in grid and flex containers.

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 baseline is start, the one for last baseline is end.

space-between

The items are evenly distributed within the alignment container along the main axis. 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.

space-around

The items are evenly distributed within the alignment container along the main axis. 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 along the main axis. 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 along the main axis 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 by max-height/max-width (or equivalent functionality), so that the combined size exactly fills the alignment container along the main axis.

Note: stretch is not supported by flexible boxes (flexbox).

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.

justify-content = 
  normal                                              |
  <content-distribution>                              |
  <overflow-position>? [ <content-position> | left | right ]  

<content-distribution> = 
  space-between  |
  space-around   |
  space-evenly   |
  stretch        

<overflow-position> = 
  unsafe  |
  safe    

<content-position> = 
  center      |
  start       |
  end         |
  flex-start  |
  flex-end    

CSS

#container {
  display: flex;
  justify-content: space-between; /* Can be changed in the live sample */
}

#container > div {
  width: 100px;
  height: 100px;
  background: linear-gradient(-45deg, #788cff, #b4c8ff);
}

Result

• CSS Flexbox Guide: Basic Concepts of Flexbox
• CSS Flexbox Guide: Aligning items in a flex container
• CSS Grid Guide: Box alignment in CSS Grid layouts
• CSS Box Alignment