Interactivity API: Mark server-side-rendered style elements for the Interactivity Router

[DAreRodz]

Summary

Note

This PR is an experiment to gather feedback on the mechanism. The client-side counterpart, the router honoring the attribute, lives in the Gutenberg repository and is not part of this PR.

During client-side navigation, the Interactivity API router diffs the incoming head against the current one and removes style assets missing from the new page. Stylesheets injected by JavaScript after load — theme switchers, consent banners, lazily loaded CSS — are never part of the server-rendered HTML, so the first navigation strips them. The router has no way to tell a server-rendered style asset from a client-injected one.

This PR marks every <style> and <link rel="stylesheet"> in the server-generated HTML with an empty data-wp-router-managed attribute. A style asset without the attribute is, by elimination, client-injected, and the router can preserve it when diffing the head. The attribute is only emitted when the router can act on it: every rendered block supports client-side navigation, a data-wp-router-region directive was processed, and clientNavigationDisabled is not set in the core/router config. Style assets inside inert <noscript> and <template> subtrees are not marked.

Implementation

Marking happens on the full page buffer with WP_HTML_Tag_Processor — per-tag APIs cannot reach inline <style> elements or the styles printed outside WP_Styles (custom CSS, WP_Font_Face, WP_Duotone, arbitrary wp_head output). Registration is lazy so response streaming is unaffected. The wp_template_enhancement_output_buffer filter is added from the router region processor: the buffer-start decision has already been made at that point, so the filter attaches to a running buffer (classic themes enable one by default since 6.9) but can never cause buffering by itself. Block themes with no active buffer start a dedicated fallback buffer from wp_head at PHP_INT_MIN priority; the block template canvas renders the whole template before wp_head, so the conditions are final there, and they are re-checked at buffer finalize in case a wp_footer callback rendered an incompatible block. Every failure path degrades to no attribute — current router behavior — and partial marking is never produced, since the router would preserve unmarked server styles as stale client ones.

See:

• Add support for dynamically injected CSS gutenberg#76031

Trac ticket:

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Fable 5, Claude Opus 5
Used for: Implementation, tests, adversarial review, and drafting this description.

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