Flex-box: Align last row to grid

Add a ::after which autofills the space. No need to pollute your HTML. Here is a codepen showing it: http://codepen.io/DanAndreasson/pen/ZQXLXj

.grid {
  display: flex;
  flex-flow: row wrap;
  justify-content: space-between;
}

.grid::after {
  content: "";
  flex: auto;
}

As other posters have mentioned - there's no clean way to left-align the last row with flexbox (at least as per the current spec)

However, for what it's worth: With the CSS Grid Layout Module this is surprisingly easy to produce:

Basically the relevant code boils down to this:

ul {
  display: grid; /* 1 */
  grid-template-columns: repeat(auto-fill, 100px); /* 2 */
  grid-gap: 1rem; /* 3 */
  justify-content: space-between; /* 4 */
}

1) Make the container element a grid container

2) Set the grid with auto columns of width 100px. (Note the use of auto-fill (as apposed to auto-fit - which (for a 1-row layout) collapses empty tracks to 0 - causing the items to expand to take up the remaining space. This would result in a justified 'space-between' layout when grid has only one row which in our case is not what we want. (check out this demo to see the difference between them)).

3) Set gaps/gutters for the grid rows and columns - here, since want a 'space-between' layout - the gap will actually be a minimum gap because it will grow as necessary.

4) Similar to flexbox.

ul {
  display: grid;
  grid-template-columns: repeat(auto-fill, 100px);
  grid-gap: 1rem;
  justify-content: space-between;
  
  /* boring properties */
  list-style: none;
  background: wheat;
  padding: 2rem;
  width: 80vw;
  margin: 0 auto;
}

li {
  height: 50px;
  border: 1px solid green;
}

<ul>
  <li></li>
  <li></li>
  <li></li>
  <li></li>
  <li></li>
  <li></li>
  <li></li>
</ul>

Codepen Demo (Resize to see the effect)