Animate to Intrinsic Sizes

Animating elements to dynamic sizes like block-size: auto or inline-size: max-content has historically required JavaScript or fragile "max-height" hacks. The interpolate-size property and calc-size() function allow the browser to natively interpolate between fixed lengths and intrinsic sizing keywords.

Implementation steps

1. Opt-in to keyword interpolation: Apply interpolate-size: allow-keywords to a parent element (typically :root) to enable transitions for properties using intrinsic keywords.
2. Define the transition: Set a transition for the sizing property (e.g., block-size, inline-size) on the target element.
3. Use intrinsic keywords: Change the sizing property to a supported intrinsic keyword—auto, min-content, max-content, fit-content, or (for flex-basis) content—during an interaction (e.g., :hover or a state class).
4. Perform calculations (Optional): Use calc-size() if you need to perform math on an intrinsic size (e.g., auto + 2rem). calc-size() also supports the any keyword for basis-agnostic calculations.

Example: Generic Expansion Pattern

You can apply this pattern to any container (like a "Show More" section or a navigation menu) to transition between a restricted height and the element's natural size.

/* Opt-in globally for all children */
:root {
  /* MANDATORY: Transitions to intrinsic keywords are disabled by default for compatibility */
  interpolate-size: allow-keywords;
}

.expandable-container {
  /* 1. Define a fixed initial size (or 0) and hide overflow */
  block-size: 100px;
  overflow: hidden;
  
  /* 2. Transition the sizing property */
  transition: block-size 0.4s ease-out;
}

.expandable-container.is-expanded {
  /* 3. Smoothly animate to the intrinsic natural height */
  block-size: auto;
}

Example: Calculated Intrinsic Inline-Size

.badge {
  inline-size: 40px;
  overflow: hidden;
  white-space: nowrap;
  transition: inline-size 0.3s ease;
}

.badge:hover {
  /* calc-size(basis, calculation) */
  /* 'size' refers to the evaluated basis (max-content in this case) */
  inline-size: calc-size(max-content, size + 20px);
}

Example: Transition from Intrinsic to Fixed

You can also animate in the opposite direction—starting from a natural size and collapsing to a specific length. This is useful for "dismissible" components.

.collapsible-alert {
  /* 1. Start with the natural content height */
  block-size: auto;
  overflow: hidden;
  transition: block-size 0.5s ease-in-out, opacity 0.5s ease;
}

.collapsible-alert.is-dismissed {
  /* 2. Smoothly collapse to zero */
  block-size: 0;
  opacity: 0;
  pointer-events: none;
}

/* MANDATORY Copy-Paste Safety: Disable sizing animations for sensitive users */
@media (prefers-reduced-motion: reduce) {
  .expandable-container,
  .badge,
  .collapsible-alert {
    transition: none !important;
  }
}

// MANDATORY Accessibility Synchronization: Ensure elements collapsed to zero dimensions are removed from the assistive technology tree, and sync aria-expanded states on triggers.
const alertElement = document.querySelector('.collapsible-alert');
alertElement.addEventListener('transitionend', (e) => {
  if (e.propertyName === 'block-size' && alertElement.classList.contains('is-dismissed')) {
    alertElement.hidden = true;
  }
});

// Example trigger syncer
const triggerBtn = document.querySelector('.accordion-trigger');
triggerBtn?.addEventListener('click', () => {
  const isExpanded = triggerBtn.getAttribute('aria-expanded') === 'true';
  triggerBtn.setAttribute('aria-expanded', !isExpanded);
});

Key constraints

• Keyword-to-Keyword Restriction: You cannot animate between two different keywords directly (e.g., from min-content to max-content). One end of the transition must be a fixed length or percentage (e.g., 0 to auto).
• Calc-size Syntax: Inside calc-size(), you cannot mix different intrinsic keywords in the same expression. The first argument (the basis) defines what size represents.
• Opt-in Requirement: Transitions to intrinsic keywords are disabled by default (numeric-only) to maintain backward compatibility. You must apply interpolate-size: allow-keywords to the element or an ancestor. calc-size() acts as a per-property override, automatically enabling interpolation whenever it is used.

Fallback strategies

interpolate-size and calc-size() are progressive enhancements. Browsers that do not support them will perform an instant jump to the target size.

• Graceful Degradation: For simple block-size: auto transitions, standard browsers will simply toggle the size instantly, which is functional but less polished.
• Manual keyword fallbacks: When using calc-size(), always provide a standard keyword fallback for older browsers, as they will discard the entire calc-size() declaration.

.card {
  block-size: auto; /* Fallback for older browsers */
  block-size: calc-size(auto, size); /* Modern browsers use this */
  transition: block-size 0.3s ease;
}