Equal Height Rows in CSS Grid Layout

#cards {
  padding: 0 7vw;
  margin-top: 3vh;
  display: grid;
  grid-template: ".   two  ." 1fr
                 "one two three" 1fr
                 "one four three" 1fr
                 ".   four  ." 1fr / 
                 1fr  1fr  1fr;
  align-items: center;
  justify-content: center;
}

.card:nth-child(1){
  grid-area: one;
}
.card:nth-child(2){
  grid-area: two;
}
.card:nth-child(3){
  grid-area: three;
}
.card:nth-child(4){
  grid-area: four;
}

.card {
  display: block;
  border: solid 5px #000;
  box-shadow: 0 20px 30px -15px hsl(215, 49%, 82%);
  border-radius: 8px;
  margin: 5%;
  overflow: hidden;
  text-align: center;
  height: 90%;
}

<div id="cards">
  <div class="card">
    <div class="card-content">
      <h2 class="card-title">Item 1</h2>
    </div>
  </div>
  <div class="card">
    <div class="card-content">
      <h2 class="card-title">Item 2</h2>
    </div>
  </div>
  <div class="card">
    <div class="card-content">
      <h2 class="card-title">Item 3</h2>
      <p>Hello world</p>
    </div>
  </div>
  <div class="card">
    <div class="card-content">
      <h2 class="card-title">Item 4</h2>
      <p>I built this two line paragraph oh yes I really want it to be two lines or yes finally.</p>
    </div>
  </div>
</div>

Equal height rows in CSS Grid Layout

Short Answer

If the goal is to create a grid with equal height rows, where the tallest cell in the grid sets the height for all rows, here's a quick and simple solution:

• Set the container to grid-auto-rows: 1fr

How it works

Grid Layout provides a unit for establishing flexible lengths in a grid container. This is the fr unit. It is designed to distribute free space in the container and is somewhat analogous to the flex-grow property in flexbox.

If you set all rows in a grid container to 1fr, let's say like this:

grid-auto-rows: 1fr;

... then all rows will be equal height.

It doesn't really make sense off-the-bat because fr is supposed to distribute free space. And if several rows have content with different heights, then when the space is distributed, some rows would be proportionally smaller and taller.

Except, buried deep in the grid spec is this little nugget:

7.2.3. Flexible Lengths: the fr
unit

...

When the available space is infinite (which happens when the grid
container’s width or height is indefinite), flex-sized (fr) grid tracks are
sized to their contents while retaining their respective proportions.

The used size of each flex-sized grid track is computed by determining
the max-content size of each flex-sized grid track and dividing that
size by the respective flex factor to determine a “hypothetical 1fr
size”.

The maximum of those is used as the resolved 1fr length (the
flex fraction), which is then multiplied by each grid track’s flex
factor to determine its final size.

So, if I'm reading this correctly, when dealing with a dynamically-sized grid (e.g., the height is indefinite), grid tracks (rows, in this case) are sized to their contents.

The height of each row is determined by the tallest (max-content) grid item.

The maximum height of those rows becomes the length of 1fr.

That's how 1fr creates equal height rows in a grid container.

Why flexbox isn't an option

As noted in the question, equal height rows are not possible with flexbox.

Flex items can be equal height on the same row, but not across multiple rows.

This behavior is defined in the flexbox spec:

6. Flex Lines

In a multi-line flex container, the cross size of each line is the minimum size necessary to contain the flex items on the line.

In other words, when there are multiple lines in a row-based flex container, the height of each line (the "cross size") is the minimum height necessary to contain the flex items on the line.

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