interpolate-size

Limited availability

This feature is not Baseline because it does not work in some of the most widely-used browsers.

Experimental: This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

The interpolate-size CSS property allows you to enable animations and transitions between a <length-percentage> value and an intrinsic size value such as auto, fit-content, or max-content.

This property is typically used to animate the width and/or height of a container between a <length-percentage> and the full size of its content (i.e. between "closed" and "open" or "hide" and "reveal" states) when animating a non-box-model CSS property, such as transform, is not a viable solution.

Note: The behavior opted-into by interpolate-size cannot be enabled by default across the web because many sites in the wild use stylesheets that assume intrinsic size values cannot be animated. Enabling it by default would cause several backwards-compatibility issues (see relevant CSS WG discussion).

Syntax

css

/* Keyword values */
interpolate-size: allow-keywords;
interpolate-size: numeric-only;

/* Global values */
interpolate-size: inherit;
interpolate-size: initial;
interpolate-size: revert;
interpolate-size: revert-layer;
interpolate-size: unset;

Values

allow-keywords: Enables interpolation between a <length-percentage> value and an intrinsic size value, to allow animation between the two.
numeric-only: The default behavior — intrinsic size values cannot be interpolated.

Description

Setting interpolate-size: allow-keywords enables interpolation between a <length-percentage> value and an intrinsic size value. Note that it does not enable animating between two intrinsic size values. One end of the animation must be a <length-percentage>.

The interpolate-size value is inherited, so animating to (or from) an intrinsic size value can be enabled for an entire document by setting it on the document root:

css

:root {
  interpolate-size: allow-keywords;
}

If you want to limit the scope, you can set it on the relevant container element. The following enables interpolating intrinsic sizes only for <main> and its descendants:

css

main {
  interpolate-size: allow-keywords;
}

Note: The calc-size() function's return values can also be interpolated. In effect, including calc-size() in a property value automatically applies interpolate-size: allow-keywords to the selection. However, because interpolate-size is inherited as explained above, it is the preferred solution for enabling intrinsic size animations in most cases. You should only use calc-size() to enable intrinsic size animations if they also require calculations.

Values that can be interpolated

The following intrinsic values can currently be opted-in to animations:

auto
min-content
max-content
fit-content
content (for containers sized using flex-basis).

Formal definition

Initial value	`numeric-only`
Applies to	all elements
Inherited	yes
Computed value	as specified
Animation type	Not animatable

Formal syntax

interpolate-size = 
  numeric-only    |
  allow-keywords

Examples

Basic `interpolate-size` usage

This example demonstrates how to set interpolate-size: allow-keywords on a document to enable animations involving an intrinsic size. The demo features a character badge/"name tag", which can be hovered or focused to reveal information about the character. The reveal is handled by a height transition between a set length and max-content.

HTML

The HTML contains a single <section> element with tabindex="0" set on it so it can receive keyboard focus. The <section> contains <header> and <main> elements, each with their own child content.

html

<section tabindex="0">
  <header>
    <h2>Tanuki</h2>
  </header>
  <main>
    <p>Tanuki is the silent phantom of MDN.</p>
    <ul>
      <li><strong>Height</strong>: 3.03m</li>
      <li><strong>Weight</strong>: 160kg</li>
      <li><strong>Tech Fu</strong>: 7</li>
      <li><strong>Bad Jokes</strong>: 9</li>
    </ul>
  </main>
</section>

CSS

* {
  box-sizing: border-box;
}

section {
  font-family: Arial, Helvetica, sans-serif;
  width: 175px;
  border-radius: 5px;
  background: #eee;
  box-shadow:
    inset 1px 1px 4px rgb(255 255 255 / 0.5),
    inset -1px -1px 4px rgb(0 0 0 / 0.5);
}

header {
  padding: 0.7rem;
  border-bottom: 2px solid #ccc;
}

main {
  padding: 10px;
}

h2 {
  margin: 0;
  font-weight: 400;
  font-size: 1.1rem;
  text-align: center;
  letter-spacing: 1px;
}

p,
li {
  font-size: 0.8rem;
  line-height: 1.5;
}

p {
  margin-top: 0;
}

In the CSS, we first set interpolate-size: allow-keywords on the :root, to enable it for the whole document.

css

:root {
  interpolate-size: allow-keywords;
}

We then set the <section>'s height to 2.5rem and overflow to hidden so only the <header> is shown by default, then specify a transition that animates the <section> height over 1 second during state change. Finally, we set the <section> height on :hover and :focus to max-content.

css

section {
  height: 2.5rem;
  overflow: hidden;
  transition: height ease 1s;
}

section:hover,
section:focus {
  height: max-content;
}

The rest of the CSS has been hidden for brevity.

Result

Try hovering over the <section> or focusing it via the keyboard — it will animate to its full height, revealing all the content.

Specifications

Specification
CSS Values and Units Module Level 5 # interpolate-size

Browser compatibility

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