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HTML 

              
                <p>
  Here's a card component whose width and height default to the size of its contents, thanks to `width: fit-content` and the default `height: auto`:
</p>

<div class="card">
  Hello world
</div>

<p>
  Here are two cards side-by-side, thanks to flexbox:
</p>

<div class="container" style="display: flex; max-width: 500px; border: 1px solid black;">
  <div class="card">
    Hello world
  </div>
  <div class="card">
    This card has more content than the first one, so much that it wraps to multiple lines. Notice how that makes the first card's height stretch to match this one.
  </div>
</div>

<p>
  So a card's height is stretchable, but can its width be, as well? Yes, with `flex-grow`!
</p>

<div class="container" style="display: flex; max-width: 500px;">
  <div class="card" style="flex-grow: 1;">
    Hello world
  </div>
  <div class="card" style="flex-grow: 1;">
    Hello world
  </div>
</div>

<p>
  Now let's try to have a grid of cards. `flex-grow` doesn't have any effect in CSS grid, so let's use `justify-self: stretch` instead:
</p>

<div class="container" style="display: grid; grid-template-columns: repeat(auto-fill, minmax(min(180px, 100%), 1fr))">
  <div class="card" style="justify-self: stretch;">
    Hello world
  </div>
  <div class="card" style="justify-self: stretch;">
    Hello world
  </div>
  <div class="card" style="justify-self: stretch;">
    Hello world
  </div>
  <div class="card" style="justify-self: stretch;">
    Hello world
  </div>
</div>

<p>
  Hmm, not quite what we expected. Why did the cards' width not stretch to fill the grid's columns?
</p>

<p>
  Because the card component has an explicit `width` of `fit-content`. Let's try to change that to `auto`:
</p>

<div class="container" style="display: grid; grid-template-columns: repeat(auto-fill, minmax(min(180px, 100%), 1fr))">
  <div class="card" style="justify-self: stretch; width: auto;">
    Hello world
  </div>
  <div class="card" style="justify-self: stretch; width: auto;">
    Hello world
  </div>
  <div class="card" style="justify-self: stretch; width: auto;">
    Hello world
  </div>
  <div class="card" style="justify-self: stretch; width: auto;">
    Hello world
  </div>
</div>

<p>
  Great, but now here's what our card looks like when used on its own:
</p>

<div class="card" style="width: auto;">
  Hello world
</div>

<p>
  Oof, that's way too wide. But if we add any `width` or `max-width` to it, we're back to our previous problem where the width is not stretchable inside a grid.
</p>

<p>
  So is there a solution? Kinda: `inline-block`. Here's a `display: inline-block; width: auto;` card on its own:
</p>

<div class="card" style="display: inline-block; width: auto;">
  Hello world
</div>

<p>
  ...in a flex container (with `flex-grow: 1`):
</p>

<div class="container" style="display: flex; max-width: 500px;">
  <div class="card" style="display: inline-block; width: auto; flex-grow: 1;">
    Hello world
  </div>
  <div class="card" style="display: inline-block; width: auto; flex-grow: 1;">
    Hello world
  </div>
</div>

<p>
  ...and in a grid (with `justify-self: stretch`):
</p>

<div class="container" style="display: grid; grid-template-columns: repeat(auto-fill, minmax(min(180px, 100%), 1fr))">
  <div class="card" style="display: inline-block; width: auto; justify-self: stretch;">
    Hello world
  </div>
  <div class="card" style="display: inline-block; width: auto; justify-self: stretch;">
    Hello world
  </div>
  <div class="card" style="display: inline-block; width: auto; justify-self: stretch;">
    Hello world
  </div>
  <div class="card" style="display: inline-block; width: auto; justify-self: stretch;">
    Hello world
  </div>
</div>

<p>
  But it is quite unexpected for a card to be `inline-block`, and can cause surprises and annoying issues with alignment. For instance, we might expect `&lt;div class="card"&gt;...&lt;/div&gt;&lt;div class="card"&gt;...&lt;/div&gt;` to render two cards on top of each other, but:
</p>

<div>
  <div class="card" style="display: inline-block; width: auto;">
    Hello world
  </div>
  <div class="card" style="display: inline-block; width: auto;">
    Hello world
  </div>
</div>

<p>
  And more importantly, what if we want the default width to be something other than `fit-content`, like `180px`?
</p>

<p>
  What I would love is a new `default-width` property. Its initial value would be `auto`, and when set to anything else, it would define the width of the element when its `width` is `auto`, while allowing it to stretch in a grid context. With this property, we could replace the card's `width: fit-content` by `default-width: fit-content` and all the examples above would work.
</p>

<p>
  Note that UAs will probably want to set `default-width` to `fit-content` (or something similar) for elements where `width: auto` seems to work differently, such as `button`, form controls, and `table` (see <a href="https://github.com/w3c/csswg-drafts/issues/7427" target="_blank">this issue</a>).
</p>

<p>
  Thanks for reading!
</p>
!

CSS 

              
                *, *::before, *::after {
  box-sizing: border-box;
}

.card {
  width: fit-content;
  padding: 10px;
  border: 1px solid black;
  border-radius: 10px;
}

p:not(:first-child) {
  margin-top: 50px;
}

.container {
  gap: 1px;
  padding: 1px;
  border: 1px solid black;
  border-radius: 12px;
}
!

JS
!
999px

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