Add cross-document View Transitions on the frontend

[westonruter]

Cross-document View Transitions enable multi-page applications (i.e. regular WordPress sites) to have a fluid navigation experience similar to a single-page app. The ​View Transitions feature plugin in Performance Lab has been very well received. (Source code is ​available on GitHub.) This ticket is to propose merging that feature plugin into core.

The feature is ​now implemented in all browsers, except for Firefox (see ​Bugzilla 1860854). Nevertheless, it is purely a progressive enhancement, the same as Speculative Loading, so Firefox users will not experience a degraded experience.

Note that the View Transitions feature goes best with Speculative Loading (initially ​merged in WP 6.8), since this reduces the latency between a link click and the transition starting. Increasing the eagerness of the speculations will improve the view transition experience (e.g. #64066).

The plugin does have a view-transitions theme support with the following defaults:

<?php
array(
        'post-selector'              => '.wp-block-post.post, article.post, body.single main',
        'global-transition-names'    => array(
                'header' => 'header',
                'main'   => 'main',
        ),
        'post-transition-names'      => array(
                '.wp-block-post-title, .entry-title'     => 'post-title',
                '.wp-post-image'                         => 'post-thumbnail',
                '.wp-block-post-content, .entry-content' => 'post-content',
        ),
        'default-animation'          => 'fade',
        'default-animation-duration' => 400,
)

There is also a user interface in the plugin to be able to configure View Transitions for themes which haven't added view-transitions theme support. This UI should be out of scope for the core merge, given the ​Decisions, not options philosophy of WordPress.

Merging this feature into core may be premature given that there are still some ​outstanding issues, so it may need to move to Future Release.

This was discussed in the ​Performance Team 2025 year-end chat.

See #64470 for implementation in WP Admin.

[joedolson]

See comment on #64470.