CSS View Transitions Module Level 1

1. Introduction

This section is non-normative.

View Transitions is a feature that allows developers to create animated transitions between visual states of the document.

1.1. Separating transitions from DOM updates

Traditionally, creating a transition between two states involves a period where both states are present in the DOM at the same time. In fact, it usually involves creating a specific DOM structure that can contain both states. If one element is "moving" between containers, that element often needs to exist outside of either container for the period of the transition, to avoid clipping from either container or ancestor elements.

This extra in-between state often results in UX and accessibility issues, as the structure of the DOM is compromised for a purely-visual effect.

View transitions avoid this troublesome in-between state by allowing the DOM to switch between states synchronously, then performing a customizable visual transition between the two states in another layer, using a static capture of the old state, and a live capture of the new state.

These captures are used in a tree of pseudo-elements (detailed in § 5.3 View transition pseudo-elements), where the old state cross-fades with the new state, while animating from the old to new size and position.

Since the captures are rendered in pseudo-elements, developers can customize the transition using familiar features, such as CSS and web animations.

The developer chooses which elements are captured independently, meaning they can be animated independently, using the view-transition-name CSS property.

1.2. Lifecycle

A successful view transition goes through the following phases:

1. Developer calls document.startViewTransition(updateCallback), which returns viewTransition, a ViewTransition.

2. Current state captured as the “old” state.

3. Rendering paused.

4. Developer’s updateCallback is called, where they update document state.

5. viewTransition.updateCallbackDone fulfills.

6. Current state captured as the “new” state.

7. Transition pseudo-elements created. See § 5.3 View transition pseudo-elements for an overview of this structure.

8. Rendering unpaused, revealing the transition pseudo-elements.

9. viewTransition.ready fulfills.

10. Pseudo-elements animate until finished.

11. Transition pseudo-elements removed.

12. viewTransition.finished fulfills.

1.3. Transitions as an enhancement

A key part of this API design is that an animated transition is an enhancement to a document state change.

That means, a failure to create a transition, which can happen due to misconfiguration or device constraints, will not prevent the developer’s UpdateCallback being called, even if it’s known in advance that the animated transition cannot happen.

For example, if the developer calls skipTransition() at the start of the lifecycle, the steps relating to the animated transition, such as creating the pseudo-elements, will not happen. However, the UpdateCallback will still be called. It’s only the transition that’s skipped, not the whole state change.

If the DOM change should also be skipped, then that should be handled by another feature. navigateEvent.signal is an example of a feature developers could use to handle this.

Although the transition API allows DOM changes to be asynchronous via the UpdateCallback, the transition API is not responsible for queuing or otherwise scheduling the DOM changes beyond the scheduling needed for the transition itself. Some asynchronous DOM changes can happen concurrently (e.g if they’re happening within independent components), whereas others need to queue, or abort an earlier change. This is best left to a feature or framework that has a more holistic view of the application.

1.4. Examples

function spaNavigate(data) {
	updateTheDOMSomehow(data);
}

function spaNavigate(data) {
	// Fallback for browsers that don’t support this API:
	if (!document.startViewTransition) {
		updateTheDOMSomehow(data);
		return;
	}

	// With a transition:
	document.startViewTransition(() => updateTheDOMSomehow(data));
}

::view-transition-old(root),
::view-transition-new(root) {
	animation-duration: 5s;
}

@keyframes fade-in {
	from { opacity: 0; }
}

@keyframes fade-out {
	to { opacity: 0; }
}

@keyframes slide-from-right {
	from { transform: translateX(30px); }
}

@keyframes slide-to-left {
	to { transform: translateX(-30px); }
}

::view-transition-old(root) {
	animation: 90ms cubic-bezier(0.4, 0, 1, 1) both fade-out,
		300ms cubic-bezier(0.4, 0, 0.2, 1) both slide-to-left;
}

::view-transition-new(root) {
	animation: 210ms cubic-bezier(0, 0, 0.2, 1) 90ms both fade-in,
		300ms cubic-bezier(0.4, 0, 0.2, 1) both slide-from-right;
}

.main-header {
	view-transition-name: main-header;
}

.main-header-text {
	view-transition-name: main-header-text;
	/* Give the element a consistent size, assuming identical text: */
	width: fit-content;
}

.sidebar {
	view-transition-name: sidebar;
}

@keyframes slide-to-right {
	to { transform: translateX(30px); }
}

/* Entry transition */
::view-transition-new(sidebar):only-child {
	animation: 300ms cubic-bezier(0, 0, 0.2, 1) both fade-in,
		300ms cubic-bezier(0.4, 0, 0.2, 1) both slide-from-right;
}

/* Exit transition */
::view-transition-old(sidebar):only-child {
	animation: 150ms cubic-bezier(0.4, 0, 1, 1) both fade-out,
		300ms cubic-bezier(0.4, 0, 0.2, 1) both slide-to-right;
}

::view-transition-image-pair(root) {
	isolation: auto;
}

::view-transition-old(root),
::view-transition-new(root) {
	animation: none;
	mix-blend-mode: normal;
}

// Store the last click event
let lastClick;
addEventListener('click', event => (lastClick = event));

function spaNavigate(data) {
	// Fallback for browsers that don’t support this API:
	if (!document.startViewTransition) {
		updateTheDOMSomehow(data);
		return;
	}

	// Get the click position, or fallback to the middle of the screen
	const x = lastClick?.clientX ?? innerWidth / 2;
	const y = lastClick?.clientY ?? innerHeight / 2;
	// Get the distance to the furthest corner
	const endRadius = Math.hypot(
		Math.max(x, innerWidth - x),
		Math.max(y, innerHeight - y)
	);

	// Create a transition:
	const transition = document.startViewTransition(() => {
		updateTheDOMSomehow(data);
	});

	// Wait for the pseudo-elements to be created:
	transition.ready.then(() => {
		// Animate the root’s new view
		document.documentElement.animate(
			{
				clipPath: [
					`circle(0 at ${x}px ${y}px)`,
					`circle(${endRadius}px at ${x}px ${y}px)`,
				],
			},
			{
				duration: 500,
				easing: 'ease-in',
				// Specify which pseudo-element to animate
				pseudoElement: '::view-transition-new(root)',
			}
		);
	});
}

2. CSS properties

2.1. view-transition-name

The view-transition-name property "names" an element as participating in a view transition.

none

The element will not participate in a view transition.

<custom-ident>

The element can participate in a view transition, as either an old or new element, with a view transition name equal to the <custom-ident>'s value.

Note: The value none is invalid as a <custom-ident>.

Note: This property causes the user-agent to both capture separate snapshots from the elements, as well as create separate pseudo-element sub-trees representing this element’s “old” and “new” states. For the purposes of this API, if one element has a transition-name "foo" in the old state, and another element has a transition-name "foo" in the new state, they are treated as representing different visual state of the same element. This may be confusing, since the elements themselves are not necessarily referring to the same object, but it is a useful model to consider them to be visual states of the same conceptual page entity, that we happen to call "element".

3. Layout and rendering changes

3.1. Named and transitioning elements

Elements have an involved in a view transition boolean, initially false.

Elements that either have a view-transition-name computed value that is not none, or are involved in a view transition, form:

• a stacking context.

• a group.

• a backdrop root.

Note: This spec uses CSS’s definition of element, which includes pseudo-elements.

3.2. Interactions during suppressed rendering

While a Document document’s transition suppressing rendering is true, pointer hit testing must target document’s document element, ignoring all other elements.

Note: This does not effect pointers that are captured.

4. User-agent styles

The global view transition user agent style sheet is a style sheet in the user-agent origin, used in all namespaces.

CSS needs a central place for an "all namespaces" style sheet.

It contains the following:

@keyframes -ua-view-transition-fade-out {
	to { opacity: 0; }
}

@keyframes -ua-view-transition-fade-in {
	from { opacity: 0; }
}

:root {
	view-transition-name: root;
}

5. Pseudo-elements

5.1. Pseudo-element root

Note: This is a general definition for trees of pseudo-elements. If other features need this behavior, these definitions will be moved to [css-pseudo-4].

A pseudo-element root is a type of tree-abiding pseudo-element that is the root in a tree of tree-abiding pseudo-elements, known as the pseudo-element tree.

The pseudo-element tree defines the document order of its descendant tree-abiding pseudo-elements.

When a pseudo-element participates in a pseudo-element tree, its originating pseudo-element is its parent.

If a descendant pseudo of a pseudo-element root has no other siblings, then :only-child matches that pseudo.

Note: This means that ::view-transition-new(ident):only-child will only select ::view-transition-new(ident) if the parent ::view-transitions-image-pair(ident) contains a single child. As in, there is no sibling ::view-transition-old(ident).

5.2. Named view-transition pseudo-elements

A named view-transition pseudo-element is a type of tree-abiding pseudo-elements.

It has a view-transition name, a string.

Their selector takes a <pt-name-selector> argument.

<pt-name-selector> = '*' | <custom-ident>

The selector matches if the <pt-name-selector> is * or matches the named view-transition pseudo-element's view-transition name.

The specificity of a view-transition selector with a <custom-ident> argument is the same as for other pseudo-elements, and is equivalent to a type selector.

The specificity of a view-transition selector with a * argument is zero.

Note: The view-transition name is set to the view-transition-name that triggered its creation.

5.3. View transition pseudo-elements

::view-transition
├─ ::view-transition-group(name)
│  └─ ::view-transition-image-pair(name)
│     ├─ ::view-transition-old(name)
│     └─ ::view-transition-new(name)
└─ …other groups…

5.3.1. ::view-transition

A tree-abiding pseudo-element that is also a pseudo-element root. Its originating element is the document’s document element.

Its containing block is the snapshot containing block.

The following is added to the global view transition user agent style sheet:

:root::view-transition {
	position: fixed;
	inset: 0;
}

5.3.2. ::view-transition-group( <pt-name-selector> )

A tree-abiding pseudo-element that is also a named view-transition pseudo-element, and participates in a pseudo-element tree.

It is selected from its ultimate originating element, the document element.

The following is added to the global view transition user agent style sheet:

:root::view-transition-group(*) {
	position: absolute;
	top: 0;
	left: 0;

	animation-duration: 0.25s;
	animation-fill-mode: both;
}

5.3.3. ::view-transition-image-pair( <pt-name-selector> )

A tree-abiding pseudo-element that is also a named view-transition pseudo-element, and participates in a pseudo-element tree.

It is selected from its ultimate originating element, the document element.

The following is added to the global view transition user agent style sheet:

:root::view-transition-image-pair(*) {
	position: absolute;
	inset: 0;

	animation-duration: inherit;
	animation-fill-mode: inherit;
}

5.3.4. ::view-transition-old( <pt-name-selector> )

A tree-abiding pseudo-element that is also a named view-transition pseudo-element, and participates in a pseudo-element tree.

It is selected from its ultimate originating element, the document element.

It is a replaced element, with natural dimensions equal to the content’s size.

Note: The image content is captured in capture the image, then set and updated in setup transition pseudo-elements and update pseudo-element styles.

The following is added to the global view transition user agent style sheet:

:root::view-transition-old(*) {
	position: absolute;
	inset-block-start: 0;
	inline-size: 100%;
	block-size: auto;

	animation-name: -ua-view-transition-fade-out;
	animation-duration: inherit;
	animation-fill-mode: inherit;
}

Note: The aim of the style is to match the element’s inline size while retaining the aspect ratio. It is also placed at the block start.

Note: Additional styles in the view transition style sheet added to animate these pseudo-elements are detailed in setup transition pseudo-elements and update pseudo-element styles.

5.3.5. ::view-transition-new( <pt-name-selector> )

Identical to ::view-transition-old(), except the following styles added to the global view transition user agent style sheet:

:root::view-transition-new(*) {
	position: absolute;
	inset-block-start: 0;
	inline-size: 100%;
	block-size: auto;

	animation-name: -ua-view-transition-fade-in;
	animation-duration: inherit;
	animation-fill-mode: inherit;
}

Note: The construction of this tree is performed in the setup transition pseudo-elements algorithm.

6. Concepts

6.1. Phases

Phases represent an ordered sequence of states. Since phases are ordered, prose can refer to phases before a particular phase, meaning they appear earlier in the sequence.

The initial phase is the first item in the sequence.

6.2. The snapshot containing block

The snapshot containing block is a rectangle that covers all areas of the window that could potentially display web content. This area is consistent regardless of root scrollbars or interactive widgets.

This means the snapshot canvas size is likely to be consistent for the document element's old image and new element.

The snapshot containing block origin refers to the top-left corner of the snapshot containing block.

The snapshot containing block size refers to the width and height of the snapshot containing block as a tuple of two numbers.

The snapshot containing block is considered to be an absolute positioning containing block and a fixed positioning containing block for ::view-transition and its descendants.

6.3. The view-transition layer stacking layer

This specification introduces a stacking layer to the Elaborate Description of Stacking Contexts.

The ::view-transition pseudo-element generates a new stacking context called the view-transition layer with the following characteristics:

• Its parent stacking context is the root stacking context.

• If the ::view-transition pseudo-element exists, a new stacking context is created for the document element and the [/=document=]'s top layer. The view-transition layer is a sibling of this stacking context.

• The view-transition layer paints after the stacking context for the document element and [/=document=]'s top layer. This includes the filters and effects that are applied to the document element.

Note: The intent of the feature is to be able to capture the contents of the page, which includes the top layer elements. In order to accomplish that, the view-transition layer cannot be a part of the captured top layer context, since that results in a circular dependency. Instead, this stacking context is a sibling of other page contents.

6.4. Captured elements

A captured element is a struct with the following:

old image

an ImageData or null. Initially null.

old width

old height

an unrestricted double, initially zero.

old transform

a <transform-function>, initially the identity transform function.

old writing-mode

Null or a writing-mode, initially null.

old direction

Null or a direction, initially null.

new element

an element or null. Initially null.

In addition, a captured element has the following style definitions:

group keyframes

A CSSKeyframesRule or null. Initially null.

group animation name rule

A CSSStyleRule or null. Initially null.

group styles rule

A CSSStyleRule or null. Initially null.

image pair isolation rule

A CSSStyleRule or null. Initially null.

view blend mode rule

A CSSStyleRule or null. Initially null.

Note: These are used to update, and later remove styles from a document's view transition style sheet.

6.5. Additions to Document

A Document additionally has:

active view transition

a ViewTransition or null. Initially null.

transition suppressing rendering

a boolean. Initially false.

view transition style sheet

a style sheet. Initially a new style sheet in the user-agent origin, ordered after the global view transition user agent style sheet.

Note: This is used to hold dynamic styles relating to transitions.

show view-transition root pseudo-element

A boolean. Initially false.

When this is true, this's active view transition's transition root pseudo-element renders as a child of this's document element, and this's document element is its originating element.

Note: The position of the transition root pseudo-element within the document element does not matter, as the transition root pseudo-element's containing block is the snapshot containing block.

7. API

7.1. Additions to Document

partial interface Document {
  ViewTransition startViewTransition(optional UpdateCallback? updateCallback = null);
};

callback UpdateCallback = Promise<any> ();

viewTransition = document.startViewTransition(updateCallback)

Starts a new view transition.

updateCallback is called asynchronously, once the current state of the document is captured. Then, when the promise returned by updateCallback fulfills, the new state of the document is captured.

updateCallback is always called, even if the transition cannot happen (e.g. due to duplicate view-transition-name values). The transition is an enhancement around the state change, so a failure to create a transition never prevents the state change. See § 1.3 Transitions as an enhancement for more details on this principle.

If the promise returned by updateCallback rejects, the transition is skipped.

7.1.1. startViewTransition()

7.2. The ViewTransition interface

[Exposed=Window]
interface ViewTransition {
  readonly attribute Promise<undefined> updateCallbackDone;
  readonly attribute Promise<undefined> ready;
  readonly attribute Promise<undefined> finished;
  undefined skipTransition();
};

Note: The ViewTransition represents and controls a single same-document transition. That is, it controls a transition where the starting and ending document are the same, possibly with changes to the document’s DOM structure.

viewTransition.updateCallbackDone

A promise that fulfills when the promise returned by updateCallback fulfills, or rejects when it rejects.

The View Transition API wraps a DOM change and creates a transition. However, sometimes you don’t care about the success/failure of the transition animation, you just want to know if and when the DOM change happens. updateCallbackDone is for that use-case.

viewTransition.ready

A promise that fulfills once the pseudo-elements for the transition are created, and the animation is about to start.

It rejects if the transition cannot begin. This can be due to misconfiguration, such as duplicate 'view-transition-name’s, or if updateCallbackDone returns a rejected promise.

The point that ready fulfills is the ideal opportunity to animate the view transition pseudo-elements with the web animation API.

viewTransition.finished

A promise that fulfills once the end state is fully visible and interactive to the user.

It only rejects if updateCallback returns a rejected promise, as this indicates the end state wasn’t created.

Otherwise, if a transition fails to begin, or is skipped (from skipTransition()), the end state is still reached, so finished fulfills.

viewTransition.skipTransition()

Immediately finish the transition, or prevent it starting.

This never prevents updateCallback being called, as the DOM change is separate to the transition. See § 1.3 Transitions as an enhancement for more details on this principle.

If this is called before ready resolves, ready will reject.

If finished hasn’t resolved, it will fulfill or reject along with updateCallbackDone.

A ViewTransition has the following:

named elements

a map, whose keys are view transition names and whose values are captured elements. Initially a new map.

phase

One of the following phases:

1. "pending-capture".

2. "update-callback-called".

3. "animating".

4. "done".

Note: For the most part, a developer using this API does not need to worry about the different phases, since they progress automatically. It is, however, important to understand what steps happen in each of the phases: when the snapshots are captured, when pseudo-element DOM is created, etc. The description of the phases below tries to be as precise as possible, with an intent to provide an unambiguous set of steps for implementors to follow in order to produce a spec-compliant implementation.

update callback

an UpdateCallback or null. Initially null.

ready promise

a Promise. Initially a new promise in this’s relevant Realm.

update callback done promise

a Promise. Initially a new promise in this’s relevant Realm.

Note: The ready promise and update callback done promise are immediately created, so rejections will cause unhandledrejections unless they’re handled, even if the getters such as updateCallbackDone are not accessed.

finished promise

a Promise. Initially a new promise in this’s relevant Realm, marked as handled.

Note: This is marked as handled to prevent duplicate unhandledrejections, as this promise only ever rejects along with the update callback done promise.

transition root pseudo-element

a ::view-transition. Initially a new ::view-transition.

initial snapshot containing block size

a tuple of two numbers (width and height), or null. Initially null.

Note: This is used to detect changes in the snapshot containing block size, which causes the transition to skip. Discussion of this behavior.

The finished getter steps are to return this’s finished promise.

The ready getter steps are to return this’s ready promise.

The updateCallbackDone getter steps are to return this’s update callback done promise.

7.2.1. skipTransition()

8. Algorithms

8.1. Monkey patches to rendering

8.2. Perform pending transition operations

8.3. Setup view transition

8.3.1. Capture the old state

8.3.2. Capture the new state

8.3.3. Setup transition pseudo-elements

8.4. Call the update callback

8.5. Skip the view transition

8.6. Capture the image

8.6.1. Capture rendering characteristics

8.7. Handle transition frame

8.8. Update pseudo-element styles

8.9. Clear view transition

Privacy Considerations

This specification introduces no new privacy considerations.

Security Considerations

The images generated using capture the image algorithm could contain cross-origin data (if the Document is embedding cross-origin resources) or sensitive information like visited links. The implementations must ensure this data can not be accessed by the Document. This should be feasible since access to this data should already be prevented in the default rendering of the Document.

Appendix A. Changes

This appendix is informative.

Changes from 2022-05-25 Working Draft

• Fix typo in ::view-transition-new user agend style sheet. See PR.

Changes from 2022-11-24 Working Draft

• Pointer events resolve to the documentElement when rendering is suppressed. See issue 7797.

• Add rendering constraints to elements participating in a transition. See issue 8139 and issue 7882.

• Remove html specifics from UA stylesheet to support ViewTransitions on SVG Documents.

• Rename updateDOMCallback to UpdateCallback. See issue 8144.

• Rename snapshot viewport to snapshot containing block.

• Skip the transition if viewport size changes. See issue 8045.

• Add support for :only-child. See issue 8057.

• Add concept of a tree of pseudo-elements under pseudo-element root. See issue 8113.

• When skipping a transition, the UpdateCallback is called in own task rather than synchronosly. See issue 7904

• When capturing images, at least the in-viewport part of the image should be captured, downscale if needed. See issue 8561.

• Applying the ink overflow to the captured image is implementation defined, and doesn’t affect the image’s natural size. See issue 8597.

• Fragmented elements don’t participate in view transitions. See issue 8339.

• Rename "snapshot root" to "snapshot containing block", and make it an absolute positioning containing block and a fixed positioning containing block for its descendants. See issue 8505.

Changes from 2022-10-25 Working Draft (FPWD)

• Add view transition style sheet concept for dynamically generated UA styles scoped to the current Document.

• Add snapshot viewport concept. See issue 7859.

• Clarify timimg for resolving/rejecting promises when skipping the transition. See issue 7956.

• Elements under a content-visiblity:auto element that skips its contents are ignored. See issue 7874.

• UA styles on the pseudo-DOM stay in sync with author DOM for any developer observable API. See issue 7812.

• Suppress rendering during updateCallback. See issue 7784.

• Changes in size/position of elements in the new Document generate new UA animation keyframes. See issue 7813.

• Scope keyframes to user agent stylesheets using -ua- prefix. See issue 7560.

• Update pseudo element names to view-transition*. See issue 7960.

• Update selector syntax for pseudo-elements. See issue 7788.

• Add sections for security/privacy considerations.