diff --git a/lib/experimental/fonts/bc-layer/class-gutenberg-fonts-api-bc-layer.php b/lib/experimental/fonts/bc-layer/class-gutenberg-fonts-api-bc-layer.php
new file mode 100644
index 0000000000000..7cf041148eedd
--- /dev/null
+++ b/lib/experimental/fonts/bc-layer/class-gutenberg-fonts-api-bc-layer.php
@@ -0,0 +1,49 @@
+<?php
+/**
+ * Fonts API BC Layer helpers.
+ *
+ * BACKPORT NOTE: Do not backport this file to Core.
+ * This file is now part of the API's Backwards-Compatibility (BC) layer.
+ *
+ * @package    Gutenberg
+ * @subpackage Fonts API
+ * @since      15.7.0
+ */
+
+/**
+ * Class Gutenberg_Fonts_API_BC_Layer
+ *
+ * BACKPORT NOTE: Do not backport this file to Core.
+ *
+ * @since X.X.X
+ * @deprecated 16.3.0 Fonts API is not supported.
+ */
+class Gutenberg_Fonts_API_BC_Layer {
+
+	/**
+	 * Determines if the given fonts array is the deprecated array structure.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Structure check is not functional.
+	 *
+	 * @return bool False.
+	 */
+	public static function is_deprecated_structure() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+		return false;
+	}
+
+	/**
+	 * Migrates deprecated fonts structure into new API data structure,
+	 * i.e. variations grouped by their font-family.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Migrate is not supported.
+	 *
+	 * @return array Empty array.
+	 */
+	public static function migrate_deprecated_structure() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+		return array();
+	}
+}
diff --git a/lib/experimental/fonts/bc-layer/class-wp-fonts-provider-local.php b/lib/experimental/fonts/bc-layer/class-wp-fonts-provider-local.php
new file mode 100644
index 0000000000000..6acdce24a7dc3
--- /dev/null
+++ b/lib/experimental/fonts/bc-layer/class-wp-fonts-provider-local.php
@@ -0,0 +1,46 @@
+<?php
+/**
+ * Webfonts API: Provider for locally-hosted fonts.
+ *
+ * @package    WordPress
+ * @subpackage Fonts API
+ * @since      X.X.X
+ */
+
+if ( class_exists( 'WP_Fonts_Provider_Local' ) ) {
+	return;
+}
+
+/**
+ * A core bundled provider for generating `@font-face` styles
+ * from locally-hosted font files.
+ *
+ * @since X.X.X
+ * @deprecated 16.3.0 Providers are not supported.
+ *                    Local provider is in WP_Font_Face.
+ */
+class WP_Fonts_Provider_Local extends WP_Fonts_Provider {
+
+	/**
+	 * The provider's unique ID.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0
+	 *
+	 * @var string
+	 */
+	protected $id = 'local';
+
+	/**
+	 * Gets the `@font-face` CSS styles for locally-hosted font files.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Get styles is not supported.
+	 *
+	 * @return string Empty string.
+	 */
+	public function get_css() {
+		_deprecated_function( __FUNCTION__, 'Gutenberg 16.3' );
+		return '';
+	}
+}
diff --git a/lib/experimental/fonts/bc-layer/class-wp-fonts-provider.php b/lib/experimental/fonts/bc-layer/class-wp-fonts-provider.php
new file mode 100644
index 0000000000000..d983b99fd9a78
--- /dev/null
+++ b/lib/experimental/fonts/bc-layer/class-wp-fonts-provider.php
@@ -0,0 +1,98 @@
+<?php
+/**
+ * Fonts API: Provider abstract class.
+ *
+ * Individual fonts providers should extend this class and implement.
+ *
+ * @package    WordPress
+ * @subpackage Fonts API
+ * @since      X.X.X
+ */
+
+if ( class_exists( 'WP_Fonts_Provider' ) ) {
+	return;
+}
+
+/**
+ * Abstract class for Fonts API providers.
+ *
+ * @since X.X.X
+ * @deprecated 16.3.0 Custom providers are not supported.
+ */
+abstract class WP_Fonts_Provider {
+
+	/**
+	 * The provider's unique ID.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0
+	 *
+	 * @var string
+	 */
+	protected $id = '';
+
+	/**
+	 * Fonts to be processed.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0
+	 *
+	 * @var array[]
+	 */
+	protected $fonts = array();
+
+	/**
+	 * Array of Font-face style tag's attribute(s)
+	 * where the key is the attribute name and the
+	 * value is its value.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0
+	 *
+	 * @var string[]
+	 */
+	protected $style_tag_atts = array();
+
+	/**
+	 * Sets this provider's fonts property.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Set is not supported.
+	 */
+	public function set_fonts() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+	}
+
+	/**
+	 * Prints the generated styles.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Use wp_print_font_faces() instead.
+	 */
+	public function print_styles() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0', 'wp_print_font_faces' );
+	}
+
+	/**
+	 * Gets the `@font-face` CSS for the provider's fonts.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0
+	 *
+	 * @return string The `@font-face` CSS.
+	 */
+	abstract public function get_css();
+
+	/**
+	 * Gets the `<style>` element for wrapping the `@font-face` CSS.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Get style element is not supported.
+	 *
+	 * @return string Empty string.
+	 */
+	protected function get_style_element() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+		return '';
+	}
+}
diff --git a/lib/experimental/fonts/bc-layer/class-wp-fonts-resolver.php b/lib/experimental/fonts/bc-layer/class-wp-fonts-resolver.php
new file mode 100644
index 0000000000000..c2702348b9523
--- /dev/null
+++ b/lib/experimental/fonts/bc-layer/class-wp-fonts-resolver.php
@@ -0,0 +1,64 @@
+<?php
+/**
+ * WP_Fonts_Resolver class.
+ *
+ * @package    WordPress
+ * @subpackage Fonts API
+ * @since      X.X.X
+ */
+
+if ( class_exists( 'WP_Fonts_Resolver' ) ) {
+	return;
+}
+
+/**
+ * The Fonts API Resolver abstracts the processing of different data sources
+ * (such as theme.json and global styles) for font interactions with the API.
+ *
+ * This class is for internal core usage and is not supposed to be used by
+ * extenders (plugins and/or themes).
+ *
+ * @access private
+ *
+ * @since X.X.X
+ * @deprecated 16.3.0 Fonts API is not supported.
+ */
+class WP_Fonts_Resolver {
+
+	/**
+	 * Enqueues user-selected fonts via global styles.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Enqueue is not supported.
+	 *
+	 * @return array Empty array.
+	 */
+	public static function enqueue_user_selected_fonts() {
+		_deprecated_function( __FUNCTION__, 'Gutenberg 16.3' );
+		return array();
+	}
+
+	/**
+	 * Register fonts defined in theme.json.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Register is not supported.
+	 */
+	public static function register_fonts_from_theme_json() {
+		_deprecated_function( __FUNCTION__, 'Gutenberg 16.3' );
+	}
+
+	/**
+	 * Add missing fonts to the global styles.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Adding fonts into theme.json theme data layer is not supported.
+	 *
+	 * @param WP_Theme_JSON_Gutenberg|WP_Theme_JSON $data The global styles.
+	 * @return WP_Theme_JSON_Gutenberg|WP_Theme_JSON Unchanged global styles.
+	 */
+	public static function add_missing_fonts_to_theme_json( $data ) {
+		_deprecated_function( __FUNCTION__, 'Gutenberg 16.3' );
+		return $data;
+	}
+}
diff --git a/lib/experimental/fonts/bc-layer/class-wp-fonts-utils.php b/lib/experimental/fonts/bc-layer/class-wp-fonts-utils.php
new file mode 100644
index 0000000000000..3290a99748349
--- /dev/null
+++ b/lib/experimental/fonts/bc-layer/class-wp-fonts-utils.php
@@ -0,0 +1,74 @@
+<?php
+/**
+ * Font API's utility helpers.
+ *
+ * @package    WordPress
+ * @subpackage Fonts API
+ * @since      X.X.X
+ */
+
+if ( class_exists( 'WP_Fonts_Utils' ) ) {
+	return;
+}
+
+/**
+ * Utility helpers for the Fonts API.
+ *
+ * @since X.X.X
+ * @deprecated 16.3.0 Fonts API is not supported.
+ */
+class WP_Fonts_Utils {
+
+	/**
+	 * Converts the given font family into a handle.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 This method is not supported.
+	 *
+	 * @return null
+	 */
+	public static function convert_font_family_into_handle() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+		return null;
+	}
+
+	/**
+	 * Converts the given variation and its font-family into a handle.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 This method is not supported.
+	 *
+	 * @return null
+	 */
+	public static function convert_variation_into_handle() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+		return null;
+	}
+
+	/**
+	 * Gets the font family from the variation.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 This method is not supported.
+	 *
+	 * @return null Null.
+	 */
+	public static function get_font_family_from_variation() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+		return null;
+	}
+
+	/**
+	 * Checks if the given input is defined, i.e. meaning is a non-empty string.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0
+	 *
+	 * @param string $input The input to check.
+	 * @return bool True when non-empty string. Else false.
+	 */
+	public static function is_defined( $input ) {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+		return ( is_string( $input ) && ! empty( $input ) );
+	}
+}
diff --git a/lib/experimental/fonts/bc-layer/class-wp-fonts.php b/lib/experimental/fonts/bc-layer/class-wp-fonts.php
new file mode 100644
index 0000000000000..9c48a7c31222b
--- /dev/null
+++ b/lib/experimental/fonts/bc-layer/class-wp-fonts.php
@@ -0,0 +1,199 @@
+<?php
+/**
+ * WP Fonts API class.
+ *
+ * @package    WordPress
+ * @subpackage Fonts API
+ * @since      X.X.X
+ */
+
+if ( class_exists( 'WP_Fonts' ) ) {
+	return;
+}
+
+/**
+ * Class WP_Fonts
+ *
+ * @since X.X.X
+ * @deprecated 16.3.0 Fonts API is not supported.
+ */
+class WP_Fonts extends WP_Dependencies {
+
+	/**
+	 * Registered "origin", indicating the font is registered in the API.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0
+	 *
+	 * @var string
+	 */
+	const REGISTERED_ORIGIN = 'gutenberg_wp_fonts_api';
+
+	/**
+	 * Constructor.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0
+	 */
+	public function __construct() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+	}
+
+	/**
+	 * Get the list of registered providers.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Providers are not supported.
+	 */
+	public function get_providers() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+	}
+
+	/**
+	 * Register a provider.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Providers are not supported.
+	 *
+	 * @return bool False.
+	 */
+	public function register_provider() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+		return false;
+	}
+
+	/**
+	 * Get the list of all registered font family handles.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Register is not supported.
+	 *
+	 * @return array Empty array.
+	 */
+	public function get_registered_font_families() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+		return array();
+	}
+
+	/**
+	 * Get the list of all registered font families and their variations.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Register is not supported.
+	 *
+	 * @return array Empty array.
+	 */
+	public function get_registered() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+		return array();
+	}
+
+	/**
+	 * Get the list of enqueued font families and their variations.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Enqueue is not supported.
+	 *
+	 * @return array Empty array.
+	 */
+	public function get_enqueued() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+		return array();
+	}
+
+	/**
+	 * Registers a font family.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Add is not supported.
+	 *
+	 * @return null Null.
+	 */
+	public function add_font_family() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+		return null;
+	}
+
+	/**
+	 * Removes a font family and all registered variations.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Remove is not supported.
+	 */
+	public function remove_font_family() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+	}
+
+	/**
+	 * Add a variation to an existing family or register family if none exists.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Add is not supported.
+	 *
+	 * @return null Null.
+	 */
+	public function add_variation() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+		return null;
+	}
+
+	/**
+	 * Removes a variation.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Remove is not supported.
+	 */
+	public function remove_variation() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+	}
+
+	/**
+	 * Processes the items and dependencies.
+	 *
+	 * Processes the items passed to it or the queue, and their dependencies.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Processing items and dependencies is not supported.
+	 *
+	 * @param string|string[]|bool $handles Optional. Items to be processed: queue (false),
+	 *                                      single item (string), or multiple items (array of strings).
+	 *                                      Default false.
+	 * @param int|false            $group   Optional. Group level: level (int), no group (false).
+	 *
+	 * @return array Empty array.
+	 */
+	public function do_items( $handles = false, $group = false ) { // phpcs:ignore VariableAnalysis.CodeAnalysis.VariableAnalysis.UnusedVariable
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+		return array();
+	}
+
+	/**
+	 * Invokes each provider to process and print its styles.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Process and print provider styles is not supported.
+	 *
+	 * @see WP_Dependencies::do_item()
+	 *
+	 * @param string    $provider_id The provider to process.
+	 * @param int|false $group       Not used.
+	 * @return bool False.
+	 */
+	public function do_item( $provider_id, $group = false ) { // phpcs:ignore VariableAnalysis.CodeAnalysis.VariableAnalysis.UnusedVariable
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+		return false;
+	}
+
+	/**
+	 * Converts the font family and its variations into theme.json structural format.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Convert is not supported.
+	 *
+	 * @return array Empty array.
+	 */
+	public function to_theme_json() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+		return array();
+	}
+}
diff --git a/lib/experimental/fonts/bc-layer/class-wp-web-fonts.php b/lib/experimental/fonts/bc-layer/class-wp-web-fonts.php
new file mode 100644
index 0000000000000..e5133eb2cb3bd
--- /dev/null
+++ b/lib/experimental/fonts/bc-layer/class-wp-web-fonts.php
@@ -0,0 +1,190 @@
+<?php
+/**
+ * Web Fonts API class.
+ *
+ * @package    WordPress
+ * @subpackage Fonts API
+ * @since      X.X.X
+ */
+
+if ( class_exists( 'WP_Web_Fonts' ) ) {
+	return;
+}
+
+/**
+ * Class WP_Web_Fonts
+ *
+ * @since 14.9.1
+ * @deprecated 15.1 Use WP_Fonts instead.
+ * @deprecated 16.3.0 Fonts API is not supported.
+ */
+class WP_Web_Fonts extends WP_Webfonts {
+
+	/**
+	 * Constructor.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0
+	 */
+	public function __construct() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+	}
+
+	/**
+	 * Get the list of registered providers.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Providers are not supported.
+	 */
+	public function get_providers() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+	}
+
+	/**
+	 * Register a provider.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Providers are not supported.
+	 *
+	 * @return bool False.
+	 */
+	public function register_provider() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+		return false;
+	}
+
+	/**
+	 * Get the list of all registered font family handles.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Register is not supported.
+	 *
+	 * @return array Empty array.
+	 */
+	public function get_registered_font_families() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+		return array();
+	}
+
+	/**
+	 * Get the list of all registered font families and their variations.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Register is not supported.
+	 *
+	 * @return array Empty array.
+	 */
+	public function get_registered() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+		return array();
+	}
+
+	/**
+	 * Get the list of enqueued font families and their variations.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Enqueue is not supported.
+	 *
+	 * @return array Empty array.
+	 */
+	public function get_enqueued() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+		return array();
+	}
+
+	/**
+	 * Registers a font family.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Add is not supported.
+	 *
+	 * @return null Null.
+	 */
+	public function add_font_family() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+		return null;
+	}
+
+	/**
+	 * Removes a font family and all registered variations.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Remove is not supported.
+	 */
+	public function remove_font_family() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+	}
+
+	/**
+	 * Add a variation to an existing family or register family if none exists.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Add is not supported.
+	 *
+	 * @return null Null.
+	 */
+	public function add_variation() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+		return null;
+	}
+
+	/**
+	 * Removes a variation.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Remove is not supported.
+	 */
+	public function remove_variation() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+	}
+
+	/**
+	 * Processes the items and dependencies.
+	 *
+	 * Processes the items passed to it or the queue, and their dependencies.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Processing items and dependencies is not supported.
+	 *
+	 * @param string|string[]|bool $handles Optional. Items to be processed: queue (false),
+	 *                                      single item (string), or multiple items (array of strings).
+	 *                                      Default false.
+	 * @param int|false            $group   Optional. Group level: level (int), no group (false).
+	 *
+	 * @return array Empty array.
+	 */
+	public function do_items( $handles = false, $group = false ) { // phpcs:ignore VariableAnalysis.CodeAnalysis.VariableAnalysis.UnusedVariable
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+		return array();
+	}
+
+	/**
+	 * Invokes each provider to process and print its styles.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Process and print provider styles is not supported.
+	 *
+	 * @see WP_Dependencies::do_item()
+	 *
+	 * @param string    $provider_id The provider to process.
+	 * @param int|false $group       Not used.
+	 * @return bool False.
+	 */
+	public function do_item( $provider_id, $group = false ) { // phpcs:ignore VariableAnalysis.CodeAnalysis.VariableAnalysis.UnusedVariable
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+		return false;
+	}
+
+	/**
+	 * Converts the font family and its variations into theme.json structural format.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Convert is not supported.
+	 *
+	 * @return array Empty array.
+	 */
+	public function to_theme_json() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+		return array();
+	}
+}
diff --git a/lib/experimental/fonts/bc-layer/class-wp-webfonts-provider-local.php b/lib/experimental/fonts/bc-layer/class-wp-webfonts-provider-local.php
new file mode 100644
index 0000000000000..9ddd813d7ccbb
--- /dev/null
+++ b/lib/experimental/fonts/bc-layer/class-wp-webfonts-provider-local.php
@@ -0,0 +1,65 @@
+<?php
+/**
+ * Fonts API: Provider for locally-hosted fonts.
+ *
+ * Part of the backwards-compatibility (BC) layer for all
+ * deprecated publicly exposed methods and functionality.
+ *
+ * This class/file will NOT be backported to Core. Rather for sites
+ * using the previous API, it exists to prevent breakages, giving
+ * developers time to upgrade their code.
+ *
+ * @package    WordPress
+ * @subpackage Fonts
+ * @since      X.X.X
+ */
+
+if ( class_exists( 'WP_Webfonts_Provider_Local' ) ) {
+	return;
+}
+
+/**
+ * A deprecated core bundled provider for generating `@font-face` styles
+ * from locally-hosted font files.
+ *
+ * BACKPORT NOTE: Do not backport this file to Core.
+ *
+ * @since X.X.X
+ * @deprecated 15.1 Use `WP_Fonts_Provider_Local` instead.
+ * @deprecated 16.3.0 Providers are not supported.
+ *                    Local provider is in WP_Font_Face.
+ */
+class WP_Webfonts_Provider_Local extends WP_Webfonts_Provider {
+
+	/**
+	 * The provider's unique ID.
+	 *
+	 * @since 6.0.0
+	 * @deprecated 16.3.0
+	 *
+	 * @var string
+	 */
+	protected $id = 'local';
+
+	/**
+	 * Constructor.
+	 *
+	 * @since 6.1.0
+	 */
+	public function __construct() {
+		_deprecated_function( __METHOD__, 'Gutenberg 15.1' );
+	}
+
+	/**
+	 * Gets the `@font-face` CSS styles for locally-hosted font files.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Get styles is not supported.
+	 *
+	 * @return string Empty string.
+	 */
+	public function get_css() {
+		_deprecated_function( __FUNCTION__, 'Gutenberg 16.3' );
+		return '';
+	}
+}
diff --git a/lib/experimental/fonts/bc-layer/class-wp-webfonts-provider.php b/lib/experimental/fonts/bc-layer/class-wp-webfonts-provider.php
new file mode 100644
index 0000000000000..d536476e91b7b
--- /dev/null
+++ b/lib/experimental/fonts/bc-layer/class-wp-webfonts-provider.php
@@ -0,0 +1,53 @@
+<?php
+/**
+ * Fonts API: Provider abstract class.
+ *
+ * Part of the backwards-compatibility (BC) layer for all
+ * deprecated publicly exposed methods and functionality.
+ *
+ * This class/file will NOT be backported to Core. Rather for sites
+ * using the previous API, it exists to prevent breakages, giving
+ * developers time to upgrade their code.
+ *
+ * @package    WordPress
+ * @subpackage Fonts API
+ * @since      X.X.X
+ */
+
+if ( class_exists( 'WP_Webfonts_Provider' ) ) {
+	return;
+}
+
+/**
+ * Deprecated abstract class for Fonts API providers.
+ *
+ * BACKPORT NOTE: Do not backport this file to Core.
+ *
+ * @since X.X.X
+ * @deprecated 15.1 Use `WP_Fonts_Provider` instead.
+ * @deprecated 16.3.0 Custom providers are not supported.
+ */
+abstract class WP_Webfonts_Provider extends WP_Fonts_Provider {
+
+	/**
+	 * Fonts to be processed.
+	 *
+	 * @since X.X.X
+	 * @deprecated 15.1 Use WP_Fonts_Provider::$fonts property instead.
+	 * @deprecated 16.3.0
+	 *
+	 * @var array[]
+	 */
+	protected $webfonts = array();
+
+	/**
+	 * Sets this provider's fonts property.
+	 *
+	 * @since X.X.X
+	 * @deprecated GB 15.1 Use WP_Fonts_Provider::set_fonts() instead.
+	 * @deprecated 16.3.0 Set is not supported.
+	 */
+	public function set_webfonts() {
+		_deprecated_function( __METHOD__, 'Gutenberg 15.1' );
+	}
+}
diff --git a/lib/experimental/fonts/bc-layer/class-wp-webfonts-utils.php b/lib/experimental/fonts/bc-layer/class-wp-webfonts-utils.php
new file mode 100644
index 0000000000000..dbe7a4325b7b7
--- /dev/null
+++ b/lib/experimental/fonts/bc-layer/class-wp-webfonts-utils.php
@@ -0,0 +1,82 @@
+<?php
+/**
+ * Webfont API's utility helpers.
+ *
+ * BACKPORT NOTE: Do not backport this file to Core.
+ * This file is now part of the API's Backwards-Compatibility (BC) layer.
+ *
+ * @package    WordPress
+ * @subpackage Fonts API
+ * @since      X.X.X
+ */
+
+if ( class_exists( 'WP_Webfonts_Utils' ) ) {
+	return;
+}
+
+/**
+ * Utility helpers for the Webfonts API.
+ *
+ * @since X.X.X
+ * @deprecated 15.1 Use WP_Fonts_Utils instead.
+ * @deprecated 16.3.0 Webfonts API is not supported.
+ */
+class WP_Webfonts_Utils {
+
+	/**
+	 * Converts the given font family into a handle.
+	 *
+	 * @since X.X.X
+	 * @deprecated 15.1 Use WP_Fonts_Utils::convert_font_family_into_handle() instead.
+	 * @deprecated 16.3.0 This method is not supported.
+	 *
+	 * @return null
+	 */
+	public static function convert_font_family_into_handle() {
+		_deprecated_function( __METHOD__, 'Gutenberg 15.1' );
+		return null;
+	}
+
+	/**
+	 * Converts the given variation and its font-family into a handle.
+	 *
+	 * @since X.X.X
+	 * @deprecated 15.1 Use WP_Fonts_Utils::convert_variation_into_handle() instead.
+	 * @deprecated 16.3.0 This method is not supported.
+	 *
+	 * @return null
+	 */
+	public static function convert_variation_into_handle() {
+		_deprecated_function( __METHOD__, 'Gutenberg 15.1' );
+		return null;
+	}
+
+	/**
+	 * Gets the font family from the variation.
+	 *
+	 * @since X.X.X
+	 * @deprecated 15.1 Use WP_Fonts_Utils::get_font_family_from_variation() instead.
+	 * @deprecated 16.3.0 This method is not supported.
+	 *
+	 * @return null
+	 */
+	public static function get_font_family_from_variation() {
+		_deprecated_function( __METHOD__, 'Gutenberg 15.1' );
+		return null;
+	}
+
+	/**
+	 * Checks if the given input is defined, i.e. meaning is a non-empty string.
+	 *
+	 * @since X.X.X
+	 * @deprecated 15.1 Use WP_Fonts_Utils::is_defined() instead.
+	 * @deprecated 16.3.0 This method is not supported.
+	 *
+	 * @param string $input The input to check.
+	 * @return bool True when non-empty string. Else false.
+	 */
+	public static function is_defined( $input ) {
+		_deprecated_function( __METHOD__, 'Gutenberg 15.1' );
+		return ( is_string( $input ) && ! empty( $input ) );
+	}
+}
diff --git a/lib/experimental/fonts/bc-layer/class-wp-webfonts.php b/lib/experimental/fonts/bc-layer/class-wp-webfonts.php
new file mode 100644
index 0000000000000..95bfa11f3d0c6
--- /dev/null
+++ b/lib/experimental/fonts/bc-layer/class-wp-webfonts.php
@@ -0,0 +1,146 @@
+<?php
+/**
+ * Webfonts API class: backwards-compatibility (BC) layer for all
+ * deprecated publicly exposed methods and functionality.
+ *
+ * This class/file will NOT be backported to Core. Rather for sites
+ * using the previous API, it exists to prevent breakages, giving
+ * developers time to upgrade their code.
+ *
+ * @package    Gutenberg
+ * @subpackage Fonts API's BC Layer
+ * @since      X.X.X
+ */
+
+if ( class_exists( 'WP_Webfonts' ) ) {
+	return;
+}
+
+/**
+ * Class WP_Webfonts
+ *
+ * @since X.X.X
+ * @deprecated 15.1 Use WP_Fonts instead.
+ * @deprecated 16.3.0 Fonts API is not supported.
+ */
+class WP_Webfonts {
+
+	/**
+	 * Constructor.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0
+	 */
+	public function __construct() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+	}
+
+	/**
+	 * Gets the font slug.
+	 *
+	 * @since X.X.X
+	 * @deprecated Use WP_Fonts_Utils::convert_font_family_into_handle() or WP_Fonts_Utils::get_font_family_from_variation().
+	 * @deprecated 16.3.0 This method is not supported.
+	 *
+	 * @return false False.
+	 */
+	public static function get_font_slug() {
+		_deprecated_function( __METHOD__, 'Gutenberg 16.3.0' );
+		return false;
+	}
+
+	/**
+	 * Initializes the API.
+	 *
+	 * @since 6.0.0
+	 * @deprecated 14.9.1 Use wp_fonts().
+	 */
+	public static function init() {
+		_deprecated_function( __METHOD__, 'GB 14.9.1', 'wp_fonts()' );
+	}
+
+	/**
+	 * Get the list of all registered font family handles.
+	 *
+	 * @since X.X.X
+	 * @deprecated GB 15.8.0 Use wp_fonts()->get_registered_font_families().
+	 * @deprecated 16.3.0 Register is not supported.
+	 *
+	 * @return array Empty array.
+	 */
+	public function get_registered_font_families() {
+		_deprecated_function( __METHOD__, 'Gutenberg 15.8.0' );
+		return array();
+	}
+
+	/**
+	 * Gets the list of registered fonts.
+	 *
+	 * @since 6.0.0
+	 * @deprecated 14.9.1 Use wp_fonts()->get_registered().
+	 * @deprecated 16.3.0 Register is not supported.
+	 *
+	 * @return array Empty array.
+	 */
+	public function get_registered_webfonts() {
+		_deprecated_function( __METHOD__, 'Gutenberg 14.9.1' );
+
+		return array();
+	}
+
+	/**
+	 * Gets the list of enqueued fonts.
+	 *
+	 * @since 6.0.0
+	 * @deprecated 14.9.1 Use wp_fonts()->get_enqueued().
+	 * @deprecated 16.3.0 Enqueue is not supported.
+	 *
+	 * @return array Empty array.
+	 */
+	public function get_enqueued_webfonts() {
+		_deprecated_function( __METHOD__, 'Gutenberg 14.9.1' );
+		return array();
+	}
+
+	/**
+	 * Gets the list of all fonts.
+	 *
+	 * @since X.X.X
+	 * @deprecated GB 14.9.1 Use wp_fonts()->get_registered().
+	 * @deprecated 16.3.0 This method is not supported.
+	 *
+	 * @return array[]
+	 */
+	public function get_all_webfonts() {
+		_deprecated_function( __METHOD__, 'Gutenberg 14.9.1', 'wp_fonts()->get_registered()' );
+		return array();
+	}
+
+	/**
+	 * Registers a webfont.
+	 *
+	 * @since 6.0.0
+	 * @deprecated GB 14.9.1 Use wp_register_fonts().
+	 * @deprecated 16.3.0 Register is not supported.
+	 *
+	 * @return bool False.
+	 */
+	public function register_webfont() {
+		_deprecated_function( __METHOD__, 'GB 14.9.1', 'wp_register_fonts()' );
+		return false;
+	}
+
+	/**
+	 * Enqueue a font-family that has been already registered.
+	 *
+	 * @since XX.X
+	 * @deprecated 14.9.1 Use wp_enqueue_fonts().
+	 * @deprecated 16.3.0 Register is not supported.
+	 *
+	 * @return bool False.
+	 */
+	public function enqueue_webfont() {
+		_deprecated_function( __METHOD__, 'Gutenberg 14.9.1' );
+		return false;
+	}
+}
diff --git a/lib/experimental/fonts/bc-layer/webfonts-deprecations.php b/lib/experimental/fonts/bc-layer/webfonts-deprecations.php
new file mode 100644
index 0000000000000..4de2e6221d50b
--- /dev/null
+++ b/lib/experimental/fonts/bc-layer/webfonts-deprecations.php
@@ -0,0 +1,291 @@
+<?php
+/**
+ * Deprecated functions provided here to give extenders time to change
+ * their plugins/themes before this API is introduced into Core.
+ *
+ * BACKPORT NOTE: Do not backport these deprecated functions to Core.
+ *
+ * @package    WordPress
+ * @subpackage Fonts API
+ * @since      X.X.X
+ */
+
+if ( ! function_exists( 'wp_webfonts' ) ) {
+	/**
+	 * Initialize $wp_webfonts if it has not been set.
+	 *
+	 * @since X.X.X
+	 * @deprecated 15.1 Use wp_fonts() instead.
+	 * @deprecated 16.3.0 No longer functional. Do not use.
+	 *
+	 * @global WP_Webfonts $wp_webfonts
+	 *
+	 * @return WP_Webfonts WP_Webfonts instance.
+	 */
+	function wp_webfonts() {
+		_deprecated_function( __FUNCTION__, 'Gutenberg 15.1' );
+
+		global $wp_webfonts;
+
+		if ( ! ( $wp_webfonts instanceof WP_Webfonts ) ) {
+			$wp_webfonts = new WP_Webfonts( wp_fonts() );
+		}
+
+		return $wp_webfonts;
+	}
+}
+
+if ( ! function_exists( 'wp_register_webfonts' ) ) {
+	/**
+	 * Registers one or more font-families and each of their variations.
+	 *
+	 * @since X.X.X
+	 * @deprecated 15.1 Use wp_register_fonts() instead.
+	 * @deprecated 16.3.0 Register is not supported.
+	 *
+	 * @return array Empty array.
+	 */
+	function wp_register_webfonts() {
+		_deprecated_function( __FUNCTION__, 'Gutenberg 15.1' );
+		return array();
+	}
+}
+
+if ( ! function_exists( 'wp_register_webfont' ) ) {
+	/**
+	 * Registers a single webfont.
+	 *
+	 * @since X.X.X
+	 * @deprecated 14.9.1 Use wp_register_fonts().
+	 * @deprecated 16.3.0 Register is not supported.
+	 *
+	 * @return bool False.
+	 */
+	function wp_register_webfont() {
+		_deprecated_function( __FUNCTION__, 'Gutenberg 14.9.1' );
+		return false;
+	}
+}
+
+if ( ! function_exists( 'wp_enqueue_webfonts' ) ) {
+	/**
+	 * Enqueues one or more font family and all of its variations.
+	 *
+	 * @since X.X.X
+	 * @deprecated 15.1 Use wp_enqueue_fonts() instead.
+	 * @deprecated 16.3.0 Enqueue is not supported.
+	 */
+	function wp_enqueue_webfonts() {
+		_deprecated_function( __FUNCTION__, 'Gutenberg 15.1' );
+	}
+}
+
+if ( ! function_exists( 'wp_enqueue_webfont' ) ) {
+	/**
+	 * Enqueue a single font family that has been registered beforehand.
+	 *
+	 * @since X.X.X
+	 * @deprecated 14.9.1 Use wp_enqueue_fonts() instead.
+	 * @deprecated 16.3.0 Enqueue is not supported.
+	 *
+	 * @return bool False.
+	 */
+	function wp_enqueue_webfont() {
+		_deprecated_function( __FUNCTION__, 'Gutenberg 14.9.1' );
+		return false;
+	}
+}
+
+if ( ! function_exists( 'wp_enqueue_webfont_variations' ) ) {
+	/**
+	 * Enqueues a specific set of web font variations.
+	 *
+	 * @since X.X.X
+	 * @deprecated 15.1 Use wp_enqueue_font_variations() instead.
+	 * @deprecated 16.3.0 No longer functional. Do not use.
+	 */
+	function wp_enqueue_webfont_variations() {
+		_deprecated_function( __FUNCTION__, 'Gutenberg 15.1' );
+	}
+}
+
+if ( ! function_exists( 'wp_deregister_webfont_variation' ) ) {
+	/**
+	 * Deregisters a font variation.
+	 *
+	 * @since 14.9.1
+	 * @deprecated 15.1 Use wp_deregister_font_variation() instead.
+	 * @deprecated 16.3.0 Deregister is not supported.
+	 */
+	function wp_deregister_webfont_variation() {
+		_deprecated_function( __FUNCTION__, 'Gutenberg 15.1' );
+	}
+}
+
+if ( ! function_exists( 'wp_get_webfont_providers' ) ) {
+	/**
+	 * Gets all registered providers.
+	 *
+	 * @since X.X.X
+	 * @deprecated 14.9.1 Use wp_fonts()->get_providers().
+	 * @deprecated 16.3.0 Providers are not supported.
+	 *
+	 * @return array Empty array.
+	 */
+	function wp_get_webfont_providers() {
+		_deprecated_function( __FUNCTION__, '14.9.1' );
+
+		return array();
+	}
+}
+
+if ( ! function_exists( 'wp_register_webfont_provider' ) ) {
+	/**
+	 * Registers a custom font service provider.
+	 *
+	 * @since X.X.X
+	 * @deprecated 15.1 Use wp_register_font_provider() instead.
+	 * @deprecated 16.3.0 Providers are not supported.
+	 *
+	 * @return bool False.
+	 */
+	function wp_register_webfont_provider() {
+		_deprecated_function( __FUNCTION__, 'GB 15.1', 'wp_register_font_provider' );
+		return false;
+	}
+}
+
+if ( ! function_exists( 'wp_print_webfonts' ) ) {
+	/**
+	 * Invokes each provider to process and print its styles.
+	 *
+	 * @since 14.9.1
+	 * @deprecated 15.1 Use wp_print_fonts() instead.
+	 * @deprecated 16.3.0 Webfonts API is not supported.
+	 *
+	 * @return array Empty array.
+	 */
+	function wp_print_webfonts() {
+		_deprecated_function( __FUNCTION__, 'Gutenberg 15.1', 'wp_print_font_faces' );
+		return array();
+	}
+}
+
+if ( ! function_exists( 'wp_fonts' ) ) {
+	/**
+	 * Initialize $wp_fonts if it has not been set.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Use Fonts Library and Font Face. Fonts API is not supported.
+	 *
+	 * @global WP_Fonts $wp_fonts
+	 *
+	 * @return WP_Fonts WP_Fonts instance.
+	 */
+	function wp_fonts() {
+		_deprecated_function( __FUNCTION__, 'Gutenberg 16.3' );
+
+		global $wp_fonts;
+
+		if ( ! ( $wp_fonts instanceof WP_Fonts ) ) {
+			$wp_fonts = new WP_Fonts();
+		}
+
+		return $wp_fonts;
+	}
+}
+
+if ( ! function_exists( 'wp_register_fonts' ) ) {
+	/**
+	 * Registers one or more font-families and each of their variations.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Register is not supported.
+	 *
+	 * @return array Empty array.
+	 */
+	function wp_register_fonts() {
+		_deprecated_function( __FUNCTION__, 'Gutenberg 16.3' );
+		return array();
+	}
+}
+
+if ( ! function_exists( 'wp_enqueue_fonts' ) ) {
+	/**
+	 * Enqueues one or more font family and all of its variations.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Enqueue is not supported.
+	 */
+	function wp_enqueue_fonts() {
+		_deprecated_function( __FUNCTION__, 'Gutenberg 16.3' );
+	}
+}
+
+if ( ! function_exists( 'wp_enqueue_font_variations' ) ) {
+	/**
+	 * Enqueues a specific set of font variations.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Enqueue is not supported.
+	 */
+	function wp_enqueue_font_variations() {
+		_deprecated_function( __FUNCTION__, 'Gutenberg 16.3' );
+	}
+}
+
+if ( ! function_exists( 'wp_deregister_font_family' ) ) {
+	/**
+	 * Deregisters a font family and all of its registered variations.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Deregister is not supported.
+	 */
+	function wp_deregister_font_family() {
+		_deprecated_function( __FUNCTION__, 'Gutenberg 16.3' );
+	}
+}
+
+if ( ! function_exists( 'wp_deregister_font_variation' ) ) {
+	/**
+	 * Deregisters a font variation.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Deregister is not supported.
+	 */
+	function wp_deregister_font_variation() {
+		_deprecated_function( __FUNCTION__, 'Gutenberg 16.3' );
+	}
+}
+
+if ( ! function_exists( 'wp_register_font_provider' ) ) {
+	/**
+	 * Registers a custom font service provider.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 Providers are not supported.
+	 *
+	 * @return bool False.
+	 */
+	function wp_register_font_provider() {
+		_deprecated_function( __FUNCTION__, 'Gutenberg 16.3' );
+		return false;
+	}
+}
+
+if ( ! function_exists( 'wp_print_fonts' ) ) {
+	/**
+	 * Invokes each provider to process and print its styles.
+	 *
+	 * @since X.X.X
+	 * @deprecated 16.3.0 For classic themes, use wp_print_font_faces(). For all other sites,
+	 *                    Font Face will automatically print all fonts in theme.json merged data layer,
+	 *                    including in theme and user activated fonts from the Fonts Library.
+	 *
+	 * @return array Empty array.
+	 */
+	function wp_print_fonts() {
+		_deprecated_function( __FUNCTION__, 'Gutenberg 16.3', 'wp_print_font_faces' );
+		return array();
+	}
+}
diff --git a/lib/load.php b/lib/load.php
index 340a900130405..7ec11252e72e8 100644
--- a/lib/load.php
+++ b/lib/load.php
@@ -142,6 +142,20 @@ function gutenberg_is_experiment_enabled( $name ) {
 		require __DIR__ . '/experimental/fonts/class-wp-font-face.php';
 		require __DIR__ . '/experimental/fonts/class-wp-font-face-resolver.php';
 		require __DIR__ . '/experimental/fonts/fonts.php';
+
+		// Load the BC Layer. Do no backport to WP Core.
+		require __DIR__ . '/experimental/fonts/bc-layer/class-wp-fonts-provider.php';
+		require __DIR__ . '/experimental/fonts/bc-layer/class-wp-fonts-utils.php';
+		require __DIR__ . '/experimental/fonts/bc-layer/class-wp-fonts.php';
+		require __DIR__ . '/experimental/fonts/bc-layer/class-wp-fonts-provider-local.php';
+		require __DIR__ . '/experimental/fonts/bc-layer/class-wp-fonts-resolver.php';
+		require __DIR__ . '/experimental/fonts/bc-layer/class-gutenberg-fonts-api-bc-layer.php';
+		require __DIR__ . '/experimental/fonts/bc-layer/webfonts-deprecations.php';
+		require __DIR__ . '/experimental/fonts/bc-layer/class-wp-webfonts-utils.php';
+		require __DIR__ . '/experimental/fonts/bc-layer/class-wp-webfonts-provider.php';
+		require __DIR__ . '/experimental/fonts/bc-layer/class-wp-webfonts-provider-local.php';
+		require __DIR__ . '/experimental/fonts/bc-layer/class-wp-webfonts.php';
+		require __DIR__ . '/experimental/fonts/bc-layer/class-wp-web-fonts.php';
 	}
 } elseif ( ! class_exists( 'WP_Fonts' ) ) {
 	// Fonts API files.