CSS View Transitions Module Level 2

1. Introduction

This section is non-normative.

View Transitions, as specified in [css-view-transitions-1], is a feature that allows developers to create animated transitions between visual states of the document.

Level 2 extends that specification, by adding the necessary API and lifecycle to enable transitions across a same-origin cross-document navigation.

1.1. Lifecycle

This section is non-normative.

A successful cross-document view transition goes through the following phases:

1. The user navigates, by clicking a link, submitting a form, traversing history using the browser UI, etc.

2. Once it’s time to unload the old document, if the navigation is same origin and the old Document has opted in to cross-document view-transitions, the old state is captured.

3. An event named reveal is fired on the new Document, with a viewTransition property, which is a ViewTransition object. This ViewTransition's updateCallbackDone is already resolved, and its captured elements are populated from the old Document.

4. Right before the new Document has the first rendering opportunity, its state is captured as the "new" state.

5. From this point forward, the transition continues as if it was a same-document transition, as per activate view transition.

1.2. Examples

// in both documents:
@view-transition {
  navigation: auto;
}

@view-transition {
  navigation: auto;
}

addEventListener('click', event => {
  sessionStorage.setItem("lastClickX", event.clientX);
  sessionStorage.setItem("lastClickY", event.clientY);
});

// This would run both on initial load and on reactivation from BFCache.
addEventListener("pagereveal", async event => {
  if (!event.viewTransition)
    return;

  const x = sessionStorage.getItem("lastClickX") ?? innerWidth / 2;
  const y = sessionStorage.getItem("lastClickY") ?? innerHeight / 2;

  const endRadius = Math.hypot(
    Math.max(x, innerWidth - x),
    Math.max(y, innerHeight - y)
  );

  await event.viewTransition.ready;

  // Animate the new document’s 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',
      pseudoElement: '::view-transition-new(root)'
    }
  );
})

2. Pseudo-classes

2.1. Active View Transition Pseudo-class :active-view-transition()'

The :active-view-transition(<vt-type-selector>) pseudo-class applies to the root element of the document, if it has a matching active view transition. It has the following syntax definition:

:active-view-transition(<vt-type-selector>)
<vt-type-selector> = '*' | <custom-ident>#

The specificity of an :active-view-transition() is one pseudo-class selector if it has value is *, and two if it has any other value.

An ::active-view-transition() pseudo-class matches the document element when it has an non-null active view transition viewTransition, for which any of the following are true:

• The <vt-type-selector> is *

• viewTransition’s active types contains at least one of the <custom-ident> values of the <vt-type-selector>.

document.startViewTransition({update: updateTheDOMSomehow, types: ["slide-in", "reverse"]});

:root:active-view-transition(slide-in) {}
:root:active-view-transition(reverse) {}
:root:active-view-transition(slide-in, reverse) {}
:root:active-view-transition(slide-in, something-else) {}
:root:active-view-transition(*) {}

document.startViewTransition(updateTheDOMSomehow);
// or
document.startViewTransition({update: updateTheDOMSomehow});

/* This would be active */
:root { }
:root:active-view-transition(*) {}

/* This would not be active */
:root:active-view-transition(slide-in) {}
:root:active-view-transition(any-type-at-all-except-star) {}

3. CSS rules

3.1. The @view-transition rule

The @view-transition rule is used by a document to indicate that cross-document navigations should setup and activate a ViewTransition. To take effect, it must be present in the old document when unloading, and in the new document when it is reveal.

3.2. @view-transition rule grammar

The @view-transition rule has the following syntax:

@view-transition {
  <declaration-rule-list>
}

Note: as per default behavior, the @view-transition rule can be nested inside a conditional group rule such as @media or @supports.

3.3. The navigation descriptor

The 'navigation' descriptor opts in to automatically starting a view transition when performing a navigation of a certain type. It needs to be enabled both in the old document (when unloading) and in the new document (when reveal).

none

There will be no transition.

auto

The transition will be enabled if the navigation is same-origin, without cross-origin redirects, and is not a reload.

3.4. The type descriptor

The 'type' descriptor sets the active types for the transition when capturing and performing the transition, equivalent to calling startViewTransition(callbackOptions) with that type.

4. API

4.1. The PageRevealEvent

Note: this should go in the HTML spec. See Issue 9315.

[Exposed=Window]
interface PageRevealEvent : Event {
  readonly attribute ViewTransition? viewTransition;
};

A PageRevealEvent has a null-or-ViewTransition view transition, initially null.

Note: this event is fired when the document is reveal.

The viewTransition getter steps are to return this's view transition.

4.2. Additions to Document

dictionary StartViewTransitionOptions {
  UpdateCallback? update = null;
  sequence<DOMString>? type = null;
};

partial interface Document {

  ViewTransition startViewTransition(optional (UpdateCallback or StartViewTransitionOptions) callbackOptions = {});
};

4.2.1. startViewTransition(callbackOptions) Method Steps

4.3. Extensions to the CSSRule interface

The CSSRule interface is extended as follows:

partial interface CSSRule {
  const unsigned short VIEW_TRANSITION_RULE = 15;
};

4.4. The CSSViewTransitionRule interface

The CSSViewTransitionRule represents a @view-transition rule.

enum ViewTransitionNavigation { "auto", "none" };
[Exposed=Window]
interface CSSViewTransitionRule : CSSRule {
  attribute ViewTransitionNavigation navigation;
  attribute DOMTokenList type;
};

5. Algorithms

5.1. Data Structures

5.1.1. Additions to Document

A Document additionally has:

pagereveal fired

a boolean, initially false.

5.1.2. Additions to ViewTransition

A ViewTransition additionally has:

is inbound cross-document transition

a boolean, initially false.

should a cross-document transition take precedent? See #9512

active types

Null or a list of strings, initially null.

5.2. Monkey patches to HTML

Prepend a step at the beginning of the task queued on navigable’s active window when applying the history step (14.11.1, here):

If changingNavigationContinuation update-only is false, then setup cross-document view-transition given oldDocument, newDocument, navigationType, and the remaining steps and return from these steps.

Note: This would wait until a transition is captured or skipped before proceeding to unloading the old document and activating the new one.

Run the following step in updating the renedering, before running the animation frame callbacks:

1. For each fully active Document doc in docs, run the reveal steps for doc.

Run the following step at the end of reactivate:

1. Set document’s pagereveal fired to false.

To reveal Document document:

1. If document’s page pagereveal fired is true, then return.

2. Let transition be the result of resolving cross-document view-transition for document.

3. Fire an event named pagereveal at document’s relevant global object, using PageRevealEvent, with view transition initialized to transition.

4. If transition is not null, then activate transition.

5. Set document’s page pagereveal fired to true.

5.3. Setting up and activating the cross-document view transition

5.3.1. Resolving the @view-transition' rule

To get the resolve @view-transition rule for a Document document:

1. Let matchingRule be the last @view-transition rule in document.

2. If matchingRule is not found, then return "skip transition".

3. If matchingRule’s navigation descriptor’s computed value is none, then return "skip transition".

4. Assert: matchingRule’s navigation descriptor’s computed value is auto.

5. Let typesDescriptor be matchingRule’s type descriptor.

6. If typesDescriptor’s computed value is none, then return an empty list.

7. Return a list of strings corresponding to typesDescriptor’s computed value.

5.3.2. Setting up the view-transition in the old Document

To setup cross-document view-transition given a Document oldDocument, a Document newDocument, a NavigationType navigationType, and onReady, which is an algorithm accepting nothing:

1. If the user agent decides to display an implementation-defined navigation experience, e.g. a gesture-based transition for a back navigation, the user agent may ignore the author-defined view transition. If that is the case, return.

2. If navigationType is reload, then return.

3. If oldDocument’s origin is not same origin as newDocument’s origin then call onReady and return.

4. If newDocument was created via cross-origin redirects is true and newDocument’s latest entry is null, then call onReady and return.

   Note: A document with a non-null latest entry is being reactivated, in which case we don’t need to check for cross-origin redirects.

5. Resolve @view-transition rule for oldDocument and let resolvedRule be the result.

6. If resolvedRule is "skip transition", then call onReady and return.

   Note: We don’t know yet if newDocument has opted in, as it might not be parsed yet. We check the opt-in for newDocument when it is reveal.

7. If oldDocument’s active view transition is not null, then skip oldDocument’s active view transition with an "AbortError" DOMException in oldDocument’s relevant Realm.

   Note: this means that any running transition would be skipped when the document is ready to unload.

8. Set document’s auto-skip view transitions to true.

   Note: this means that calling startViewTransition() while capturing the old document for a cross-document view-transition would run the callback but skip the animation.

9. Let outboundTransition be a new ViewTransition object in oldDocument’s relevant Realm, whose active types is resolvedRule, and whose process old state captured is set to the following steps:

   should we check for the opt-in again, in case there was a CSSOM change in a requestAnimationFrame callback?

   1. If outboundTransition’s phase is "done", then call onReady and return.

   2. Assert: outboundTransition’s phase is "pending-capture".

   3. Clear view transition outboundTransition.

      Note: The ViewTransition object on the old Document should be destroyed after its state has been copied to the new Document below. We explicitly clear it here since the old Document may be cached by the UA.

   4. Set document’s auto-skip view transitions to false.

   5. Queue a global task on the DOM manipulation task source given newDocument’s relevant global object, to perform the following steps:

      1. Let inboundTransition be a new ViewTransition in newDocument’s relevant Realm, whose named elements is outboundTransition’s named elements, initial snapshot containing block size is outboundTransition’s initial snapshot containing block size, and whose is inbound cross-document transition is true.

      2. Let newDocument’s active view transition be inboundTransition.

      3. Call the update callback for inboundTransition.

      4. Call onReady.

      5. At any given time, the UA may decide to skip inboundTransition, e.g. after an implementation-defined timeout. To do so, the UA should queue a global task on the DOM manipulation task source given document’s relevant global object to perform the following step: If transition’s phase is not "done", then skip the view transition transition with a "TimeoutError" DOMException.

   Note: outboundTransition is not exposed to JavaScript, it is used only for capturing the state of the old document.

10. Set oldDocument’s active view transition to outboundTransition.

    Note: The process continues in setup view transition, via perform pending transition operations.

11. The user agent should display the currently displayed frame until either:

    • newDocument is revealed

    • its active view transition's phase is "done".

    Note: this is to ensure that there are no unintended flashes between displaying the old and new state, to keep the transition smooth.

5.3.3. Accessing the view-transition in the new Document

To resolve cross-document view-transition for Document document:

1. Let transition be document’s active view transition.

2. If transition is null or transition’s is inbound cross-document transition is false, then return null.

   Note: transition’s is inbound cross-document transition would be false if a same-document transition was started before the page was reveal.

3. Resolve @view-transition rule for document and let resolvedRule be the result.

4. If resolvedRule is "skip transition", then skip transition and return null.

5. Set transition’s active types to resolvedRule.

6. Return transition.

Privacy Considerations

This specification introduces no new privacy considerations.

Security Considerations

To prevent cross-origin issues, at this point cross-document view transitions can only be enabled for same-origin navigations. As discussed in WICG/view-transitions#200, this still presents two potential threats:

1. The cross-origin isolated capability in both documents might be different. This can cause a situation where a Document that is cross-origin isolated can read image data from a document that is not cross-origin isolated. This is already mitigated in [[css-view-transitions-1#sec], as the same restriction applies for captured cross-origin iframes.

2. A same-origin navigation might still occur via a cross-origin redirect, e.g. https://example.com links to https://auth-provider.com/ which redirects back to https://example.com/loggedin.

   This can cause a (minor) situation where the cross-origin party would redirect the user to an unexpected first-party URL, causing an unexpected transition and obfuscating that fact that there was a redirect. To mitigate this, currently view transitions are disabled for navigations if the Document was created via cross-origin redirects. Note that this check doesn’t apply when the Document is being reactivated, as in that case the cross-origin redirect has already taken place.

   Note: this only applies to server-side redirects. A client-side redirect, e.g. using [^meta/http-equiv/refresh^], is equivalent to a new navigation.

3. This feature exposes more information to CSS, as so far CSS was not aware of anything navigation-related. This can raise concerns around safety 3rd-party CSS. However, as a general rule, 3rd-party stylesheets should come from trusted sources to begin with, as CSS can learn about the document or change it in many ways.

See Issue #8684 and WICG/view-transitions#200 for detailed discussion.