WordPress 6.6 Field Guide

This guide outlines major developer features and breaking changes in 6.6 and is published in the Release Candidaterelease candidate One of the final stages in the version release cycle, this version signals the potential to be a final release to the public. Also see alpha (beta). cycle to help inform WordPress extending developers, CoreCore Core is the set of software required to run WordPress. The Core Development Team builds WordPress. developers, and others.

There are almost 299 Core TRACTrac An open source project by Edgewall Software that serves as a bug tracker and project management tool for WordPress. tickets included in WordPress 6.6, 108 of which are enhancements and feature requests, 171 bug fixes, and 10 other blessed tasks. This time, 16 tickets focused on performance, 24 on accessibility, and 15 on modernizing code and applying coding standards. Changes in 6.6 are spread across 40 Core components.

This release includes 392 enhancements, 462 bugbug A bug is an error or unexpected result. Performance improvements, code optimization, and are considered enhancements, not defects. After feature freeze, only bugs are dealt with, with regressions (adverse changes from the previous version) being the highest priority. fixes, and 46 accessibilityAccessibility Accessibility (commonly shortened to a11y) refers to the design of products, devices, services, or environments for people with disabilities. The concept of accessible design ensures both “direct access” (i.e. unassisted) and “indirect access” meaning compatibility with a person’s assistive technology (for example, computer screen readers). (https://en.wikipedia.org/wiki/Accessibility) improvements for the Block Editor (a.k.a. gutenbergGutenberg The Gutenberg project is the new Editor Interface for WordPress. The editor improves the process and experience of creating new content, making writing rich content much simpler. It uses ‘blocks’ to add richness rather than shortcodes, custom HTML etc. https://wordpress.org/gutenberg/).

Below is the breakdown of the most important developer-related changes included in WordPress 6.6.

Table of contents

BlockBlock Block is the abstract term used to describe units of markup that, composed together, form the content or layout of a webpage using the WordPress editor. The idea combines concepts of what in the past may have achieved with shortcodes, custom HTML, and embed discovery into a single consistent API and user experience. Editor

WordPress 6.6 brings 8 Gutenberg releases into core – 17.8, 17.9, 18.0, 18.1, 18.2, 18.3, 18.4, and 18.5. The Block Editor receives several improvements related to the ReactReact React is a JavaScript library that makes it easy to reason about, construct, and maintain stateless and stateful user interfaces. https://reactjs.org/. library, the Block APIAPI An API or Application Programming Interface is a software intermediary that allows programs to interact with each other and share data in limited, clearly defined ways., Themes, and more.


A new version of React and the new JSX transform is available in WordPress 6.6.

Block API

Some Block API improvements available in WordPress 6.6 include:

  • Unification of slots and extensibility APIs between the post and site editors
  • Improvements on isActive property of Block variations
  • Improvements on some core blocks


WordPress 6.6 introduces several theme-related updates, including:

  • A new version 3 of theme.json
  • Uniform CSSCSS Cascading Style Sheets. specificity applied across core styles
  • Introduction of section styles to streamline the styling of blocks and their internal elements
  • Additional features for the grid layout type in blocks
  • Capability to define site-wide background images in theme.json and the Site Editor

Miscellaneous Block Editor Changes

Some other updates to the Block Editor are also included in WordPress 6.6:

A table on design tools supported per block at WordPress 6.6 has been published as a reference.

Interactivity API

WordPress 6.6 includes updates for the Interactivity API, such as new async directives, support for derived state props from PHPPHP The web scripting language in which WordPress is primarily architected. WordPress requires PHP 5.6.20 or higher, integration with Preact Devtools, and new warning messages available when the SCRIPT_DEBUG configuration constant is enabled.

HTMLHTML HyperText Markup Language. The semantic scripting language primarily used for outputting content in web browsers. API

WordPress 6.6 includes a helpful maintenance release to the HTML API. This work includes a few new features and a major improvement to the HTML Processor’s usability. This continues the fast-paced development since WordPress 6.5.

There’s also a new data structure coming in WordPress 6.6 for the HTML API: the WP_Token_Map.

Options API

Several changes have been made to the Options API to support an optimization for the autoloading behavior, and to create a way to apply further optimizations going forward.

PHP Support

Support for PHP 7.0 and 7.1 is dropped in WordPress 6.6.

I18Ni18n Internationalization, or the act of writing and preparing code to be fully translatable into other languages. Also see localization. Often written with a lowercase i so it is not confused with a lowercase L or the numeral 1. Often an acquired skill.

Various internationalization (i18n) improvements are in WordPress 6.6, including:

Miscellaneous Developer Changes

Some other changes included in WordPress 6.6 are:

New/Modified HooksHooks In WordPress theme and development, hooks are functions that can be applied to an action or a Filter in WordPress. Actions are functions performed when a certain event occurs in WordPress. Filters allow you to modify certain functions. Arguments used to hook both filters and actions look the same.

For a list of all new and updated Functions/Hooks/Classes/Methods in WP 6.6, please see this page on Developer Resources after the release: https://developer.wordpress.org/reference/since/6.6.0/.

Modified FilterFilter Filters are one of the two types of Hooks https://codex.wordpress.org/Plugin_API/Hooks. They provide a way for functions to modify data of other functions. They are the counterpart to Actions. Unlike Actions, filters are meant to work in an isolated manner, and should never have side effects such as affecting global variables and output. Hooks

New Filter Hooks

  • interactivity_process_directives [58234]
  • wp_theme_files_cache_ttl [58025]
  • wp_default_autoload_value [57920]
  • wp_max_autoloaded_option_size [57920]
  • wp_autoload_values_to_autoload [57920]
  • lang_dir_for_domain [58236]
  • activate_tinymce_for_media_description [58372]
  • site_status_autoloaded_options_action_to_perform [58332]
  • site_status_autoloaded_options_size_limit [58332]
  • site_status_autoloaded_options_limit_description [58332]

New Action Hooks

  • delete_post_{$post->post_type} [57853]
  • deleted_post_{$post->post_type} [57853]

Props to @sabernhardt, @milana_cap, @stevenlinx and @bph for review.

#6-6, #dev-notes, #field-guide