Use Custom HTML block for freeform content behind an experiment

[tyxla]

What?

Adds a Gutenberg experiment that switches the freeform content handler — the block used to hold non-block (freeform) content that has no block delimiters — from the Classic block (core/freeform) to the Custom HTML block (core/html).

Why?

Exploring whether core/html is a better default home for freeform content than the Classic block (core/freeform). Wrapping it as an opt-in experiment so the behavior change is gated and easy to evaluate.

Part of #78067.

How?

• packages/block-library/src/index.js — In registerCoreBlocks, the freeform content handler now checks a new window.__experimentalEnableHtmlFreeformHandler global first; when set, it calls setFreeformContentHandlerName( html.name ) instead of the Classic-block path.
• lib/experimental/editor-settings.php — Bridges the experiment toggle to that JS global (same pattern as the existing form/block experiments).
• lib/experimental/experiments/load.php — Registers the gutenberg-html-freeform-handler experiment in the Blocks group on the Experiments screen.

This reuses a known-good combination: the widgets editors (edit-widgets, customize-widgets) already use setFreeformContentHandlerName( 'core/html' ), and core/html is always registered, so no extra guard is needed.

Testing Instructions

1. Build (npm run build or npm start).
2. Go to Gutenberg → Experiments and enable "Custom HTML for freeform content".
3. Edit a post containing non-block / freeform content (e.g. seed raw HTML via the Code editor) and confirm it is handled by the Custom HTML block rather than the Classic block.
4. Disable the experiment and confirm the prior behavior (Classic block) is restored.

Notes

• Default-off; behavior is unchanged unless the experiment is enabled.
• When enabled, the experiment switches to core/html unconditionally — including when wp.oldEditor is present — so the Classic-block fallback is disabled while the experiment is on.

[github-actions]

Size Change: +26 B (0%)

Total Size: 7.51 MB

📦 View Changed

Filename	Size	Change
build/scripts/block-library/index.min.js	324 kB	+26 B (+0.01%)

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