A grid layout with responsive squares

Solution 1:

The padding-bottom trick is the most used to accomplish that.

You can combine it with both Flexbox and CSS Grid, and since using percent for margin/padding gives inconsistent result for flex/grid items (on older browser versions, see edit note below), one can add an extra wrapper, or like here, using a pseudo, so the element with percent is not the flex/grid item.

Edit: Note, there's an update made to the specs., that now should give consistent result when used on flex/grid items. Be aware though, the issue still occurs on older versions.

Note, if you will add content to the content element, it need to be position absolute to keep the square's aspect ratio.

Fiddle demo - Flexbox

Edit 2: In a comment I were asked how to have a centered text, so I added that in below snippet.

.square-container {
  display: flex;
  flex-wrap: wrap;
}

.square {
  position: relative;
  flex-basis: calc(33.333% - 10px);
  margin: 5px;
  border: 1px solid;
  box-sizing: border-box;
}

.square::before {
  content: '';
  display: block;
  padding-top: 100%;
}

.square .content {
  position: absolute;
  top: 0; left: 0;
  height: 100%;
  width: 100%;

  display: flex;               /* added for centered text */
  justify-content: center;     /* added for centered text */
  align-items: center;         /* added for centered text */
}

<div class="square-container">

  <div class="square">
    <div class="content">
      <span>Some centered text</span>
    </div>
  </div>

  <div class="square">
    <div class="content spread">
    </div>
  </div>

  <div class="square">
    <div class="content column">
    </div>
  </div>

  <div class="square">
    <div class="content spread">
    </div>
  </div>

  <div class="square">
    <div class="content column">
    </div>
  </div>

</div>

CSS Grid version

.square-container {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(30%, 1fr));
  grid-gap: 10px;
}

.square {
  position: relative;
  border: 1px solid;
  box-sizing: border-box;
}

.square::before {
  content: '';
  display: block;
  padding-top: 100%;
}

.square .content {
  position: absolute;
  top: 0; left: 0;
  height: 100%;
  width: 100%;
}

<div class="square-container">

  <div class="square">
    <div class="content">
    </div>
  </div>

  <div class="square">
    <div class="content spread">
    </div>
  </div>

  <div class="square">
    <div class="content column">
    </div>
  </div>

  <div class="square">
    <div class="content spread">
    </div>
  </div>

  <div class="square">
    <div class="content column">
    </div>
  </div>

</div>

Solution 2:

Try using viewport percentage units.

jsFiddle

.square-container {
  display: grid;
  grid-template-columns: repeat(3, 30vw);
  grid-template-rows: 30vw;
  grid-gap: 2.5vw;
  padding: 2.5vw;
  background-color: gray;
}

.square {
  background-color: lightgreen;
}

body {
  margin: 0; /* remove default margins */
}

<div class="square-container">
  <div class="square">
    <div class="content"></div>
  </div>
  <div class="square">
    <div class="content spread"></div>
  </div>
  <div class="square">
    <div class="content column"></div>
  </div>
</div>

From the spec:

5.1.2. Viewport-percentage lengths: the vw, vh, vmin, vmax units

The viewport-percentage lengths are relative to the size of the initial containing block. When the height or width of the initial containing block is changed, they are scaled accordingly.

vw unit - Equal to 1% of the width of the initial containing block.

vh unit - Equal to 1% of the height of the initial containing block.

vmin unit - Equal to the smaller of vw or vh.

vmax unit - Equal to the larger of vw or vh.

Solution 3:

You can use the fact that padding is calculated based on the width and set padding-top: 100% directly to the square grid items (the grid items would be square now).

2019 update

Note that for flex items as well as grid items earlier this doesn't used to work - see the post linked in the comments to this answer:

Why doesn't percentage padding / margin work on flex items in Firefox and Edge?

Now that there is a consensus between browsers (newer versions) to have the same behaviour for padding for flex items and grid items, you can use this solution.

See demo below:

.square-container {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(30%, 1fr));
  grid-gap: 10px;
}

.square {
  background: cadetblue;
  padding-top: 100%; /* padding trick directly on the grid item */
  box-sizing: border-box;
  position: relative;
}

.square .content { /* absolutely positioned */
  position: absolute;
  top: 0;
  right:0;
  left: 0;
  bottom: 0;
}

<div class="square-container">
  <div class="square">
    <div class="content"> some content here</div>
  </div>
  <div class="square">
    <div class="content"> some content here</div>
  </div>
  <div class="square">
    <div class="content"> some content here</div>
  </div>
  <div class="square">
    <div class="content"> some content here</div>
  </div>
  <div class="square">
    <div class="content column">some content here and there is a lot of text here</div>
  </div>
  <div class="square">
    <div class="content spread">text</div>
  </div>
  <div class="square">
    <div class="content column">some text here</div>
  </div>
</div>