Reverse order of columns in CSS Grid Layout

As the Grid auto-placement algorithm lays out items in the container, it uses next available empty cells (source).

In your source code the A element comes before the B element:

<div id="container" class="reverse" style="width: 800px;">
   <div class="a">A</div>
   <div class="b">B</div>
</div>

Therefore, the grid container first places A, then uses the next available space to place B.

By default, the auto-placement algorithm looks linearly through the grid without backtracking; if it has to skip some empty spaces to place a larger item, it will not return to fill those spaces. To change this behavior, specify the dense keyword in grid-auto-flow.

^{http://www.w3.org/TR/css3-grid-layout/#common-uses-auto-placement}

grid-auto-flow: dense

One solution to this problem (as you have noted) is to override the default grid-auto-flow: row with grid-auto-flow: dense.

With grid-auto-flow: dense, the Grid auto-placement algorithm will look to back-fill unoccupied cells with items that fit.

#container {
  display: grid;
  grid-template-columns: 240px 1fr;
  grid-auto-flow: dense; /* NEW */
}

7.7. Automatic Placement: the grid-auto-flow property

Grid items that aren’t explicitly placed are automatically placed into an unoccupied space in the grid container by the auto-placement algorithm.

grid-auto-flow controls how the auto-placement algorithm works, specifying exactly how auto-placed items get flowed into the grid.

dense

If specified, the auto-placement algorithm uses a “dense” packing algorithm, which attempts to fill in holes earlier in the grid if smaller items come up later. This may cause items to appear out-of-order, when doing so would fill in holes left by larger items.

#container {
  display: grid;
  grid-template-columns: 240px 1fr;
  grid-auto-flow: dense; /* NEW */
}

.a {
  background: yellow;
}

.b {
  background: blue;
  color: white;
}

#container>.a {
  grid-column: 1;
}

#container>.b {
  grid-column: 2;
}

#container.reverse>.a {
  grid-column: 2;
}

#container.reverse>.b {
  grid-row: 1;
  grid-column: 1;
}

<div id="container" class="reverse" style="width: 800px;">
  <div class="a">A</div>
  <div class="b">B</div>
</div>

grid-row: 1

Another solution would be to simply define the row for the second item.

#container>.b {
  grid-column: 2;
  grid-row: 1; /* NEW */
}

#container {
  display: grid;
  grid-template-columns: 240px 1fr;
}

.a {
  background: yellow;
}

.b {
  background: blue;
  color: white;
}

#container>.a {
  grid-column: 1;
}

#container>.b {
  grid-column: 2;
  grid-row: 1; /* NEW */
}

#container.reverse>.a {
  grid-column: 2;
}

#container.reverse>.b {
  grid-row: 1;
  grid-column: 1;
}

<div id="container" class="reverse" style="width: 800px;">
  <div class="a">A</div>
  <div class="b">B</div>
</div>

The simplest way is to add order: 1 to element B or order: -1 to element A in .reverse

It's also correct CSS rather than hack-y