CSS Only Solution to Set Multiple "Same Height" Row Sections on a Responsive Grid

.row {
  display: flex; /* equal height of the children */
}

.col {
  flex: 1; /* additionally, equal width */
  
  padding: 1em;
  border: solid;
}

<div class="row">
  <div class="col">Lorem ipsum dolor sit amet, consectetur adipisicing elit.</div>
  <div class="col">Lorem ipsum dolor sit amet, consectetur adipisicing elit. Ad omnis quae expedita ipsum nobis praesentium velit animi minus amet perspiciatis laboriosam similique debitis iste ratione nemo ea at corporis aliquam.</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.

Equal height of elements inside grid item with CSS grid layout

Is it even possible?

tldr; Yes.

Codepen Demo #1

Codepen Demo # 2 (Uses SASS and is configurable)

There is actually a way to pull this off with css grids & without changing any markup!

We could 'flatten' the structure with CSS such that all the components of all the articles are part of just one CSS grid - the article container.

This is possible without even changing the current markup by setting the articles with display: contents

display: contents (caniuse)

From Caniuse:

display: contents causes an element's children to appear as if they
were direct children of the element's parent, ignoring the element
itself. This can be useful when a wrapper element should be ignored
when using CSS grid or similar layout techniques.

So if we set the articles with display: contents

.container article {
  display: contents;
}

Now all of the headers, sections and footer become (direct) grid items (of the container - which has display:grid) which we can arrange using the grid-template-areas property.

.container {
  display: grid;
  grid-column-gap: 1em; /* horizontal gap between articles */
  grid-template-columns: repeat(3, 1fr);

  grid-template-areas: "header1 header2 header3" 
                       "section1 section2 section3"
                       "footer1 footer2 footer3"
                       "header4 header5 header6" 
                       "section4 section5 section6"
                       "footer4 footer5 footer6"
}

Since each header/section/footer take up exactly one cell - this forces them to take up the same vertical height. So e.g. header1,header2 and header3 will all have the same height regardless of their content.

Now set the grid-area properties on each of the cells.

article:nth-child(1) header {
  grid-area: header1;
}
article:nth-child(2) header {
  grid-area: header2;
}
article:nth-child(3) header {
  grid-area: header3;
}
article:nth-child(4) header {
  grid-area: header4;
}
article:nth-child(1) section {
  grid-area: section1;
}
...
article:nth-child(4) section {
  grid-area: section4;
}
article:nth-child(1) footer {
  grid-area: footer1;
}
...
article:nth-child(4) footer {
  grid-area: footer4;
}

Finally, set a vertical gap between each row of articles (starting from the second row of articles):

article:nth-child(n + 4) header {
  margin-top: 1em;
}

Demo:

body {  width: 100%;  max-width: 1024px;  margin: auto;}
.container {  display: grid;  grid-column-gap: 1em;  grid-template-columns: repeat(3, 1fr);  grid-template-areas: "header1 header2 header3"                       "section1 section2 section3"                        "footer1 footer2 footer3"                        "header4 header5 header6"                       "section4 section5 section6"                        "footer4 footer5 footer6"}
.container article {  display: contents;}
article header {  background-color: #eeeeee;}
article section {  background-color: #cccccc;}
article footer {  background-color: #dddddd;}
article:nth-child(n + 4) header {  margin-top: 1em;}
article:nth-child(1) header {  grid-area: header1;}article:nth-child(2) header {  grid-area: header2;}article:nth-child(3) header {  grid-area: header3;}article:nth-child(4) header {  grid-area: header4;}article:nth-child(1) section {  grid-area: section1;}article:nth-child(2) section {  grid-area: section2;}article:nth-child(3) section {  grid-area: section3;}article:nth-child(4) section {  grid-area: section4;}article:nth-child(1) footer {  grid-area: footer1;}article:nth-child(2) footer {  grid-area: footer2;}article:nth-child(3) footer {  grid-area: footer3;}article:nth-child(4) footer {  grid-area: footer4;}

<div class="container">    <article>        <header>            <h2>Header</h2>            <h2>Header</h2>        </header>        <section>            <p>Content</p>        </section>        <footer>            <p>Footer</p>        </footer>    </article>    <article>        <header>            <h2>Header</h2>        </header>        <section>            <p>Content</p>            <p>Content</p>            <p>Content</p>            <p>Content</p>            <p>Content</p>        </section>        <footer>            <p>Footer</p>            <p>Footer</p>        </footer>    </article>    <article>        <header>            <h2>Header</h2>        </header>        <section>            <p>Content</p>            <p>Content</p>            <p>Content</p>        </section>        <footer>            <p>Footer</p>        </footer>    </article>    <article>        <header>            <h2>Header</h2>        </header>        <section>            <p>Content</p>            <p>Content</p>            <p>Content</p>            <p>Content</p>        </section>        <footer>            <p>Footer</p>            <p>Footer</p>        </footer>    </article></div>

* {  box-sizing: border-box;}body {  margin: 0;}.wrapper {  display: flex;  flex-flow: row wrap;}.wrapper > div {  border: 1px solid red;}

<div class="wrapper">  <div>    some text here some text here  </div>
  <div>    some text here    <br/>more text here  </div>
  <div>    some text here    <br/>more text here and some more and some more  </div>
  <div>    some text here    <br/>more text here    <br/>more text here  </div>
</div>