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:
@auto-view-transitions {
  same-origin: enable;
}

@auto-view-transitions {
  same-origin: enable;
}

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("reveal", 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. CSS rules

2.1. The @auto-view-transition rule

The @auto-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 being revealed.

2.2. @auto-view-transition rule grammar

@auto-view-transition rules are parsed according to the following grammar, plus the additional rules noted below:

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

2.3. The same-origin property

The 'same-origin' property opts in to automatically performing a view transition when performing a same origin navigation. It needs to be enabled both in the old document (when unloading) and in the new document (when ready to render).

disabled

There will be no transition.

enabled

The transition will be enabled if the navigation is same-origin, without cross-origin redirects.

3. API

3.1. The PageRevealEvent

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

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

Note: this event is fired when revealing a document.

The viewTransition getter steps are to return the inbound cross-document view-transition for this’s relevant global object’s associated document.

4. Algorithms

4.1. Data Structures

4.1.1. Additions to Document

A Document additionally has:

is revealed

a boolean, initially false.

4.1.2. Additions to ViewTransition ## {#view-transitions-extension}

A ViewTransition additionally has:

is inbound cross-document transition

a boolean, initially false.

4.2. 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 outbound cross-document view transition given oldDocument, newDocument 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, reveal doc.

Run the following step at the end of reactivate:

1. Set document’s is revealed to false.

To reveal Document document:

1. If document’s is revealed is true, then return.

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

3. If transition is not null and document does not opt in to cross-document view transitions, then skip transition and set transition to null.

4. Fire a new event named reveal on document’s relevant global object, using PageRevealEvent.

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

6. Set document’s is revealed to true.

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

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

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

2. 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.

3. If oldDocument does not opt in to cross-document view transitions, 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 revealed.

4. 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.

5. Let outboundTransition be a new ViewTransition object in oldDocument’s relevant Realm, 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. Queue a global task on the DOM manipulation task source given newDocument’s relevant global object, to perform the following step:

      1. Let newDocument’s active view transition 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. Call the update callback for newDocument’s active view transition.

      3. Call onReady.

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

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

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

To get the inbound cross-document view-transition for a 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 revealed.

3. Return transition.

A Document document is said to opt in to cross-document view transitions if the computed value of same-origin is enabled.

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.