Add a client-side font face resolver in the editor

packages/block-editor/src/components/editor-styles/index.js

@@ -18,6 +18,7 @@ import { useSelect } from '@wordpress/data';
 import transformStyles from '../../utils/transform-styles';
 import { store as blockEditorStore } from '../../store';
 import { unlock } from '../../lock-unlock';
+import useEditorFontsResolver from '../use-editor-fonts-resolver';
 
 extend( [ namesPlugin, a11yPlugin ] );
 
@@ -105,6 +106,9 @@ function EditorStyles( { styles, scope, transformOptions } ) {
 			<style
 				ref={ useDarkThemeBodyClassName( transformedStyles, scope ) }
 			/>
+
+			<style ref={ useEditorFontsResolver() } />
+
 			{ transformedStyles.map( ( css, index ) => (
 				<style key={ index }>{ css }</style>
 			) ) }

packages/block-editor/src/components/use-editor-fonts-resolver/index.js

@@ -0,0 +1,72 @@
+/**
+ * WordPress dependencies
+ */
+import { useState, useMemo, useCallback } from '@wordpress/element';
+import { useSelect } from '@wordpress/data';
+
+/**
+ * Internal dependencies
+ */
+import { getDisplaySrcFromFontFace, loadFontFaceInBrowser } from './utils';
+import { store as editorStore } from '../../store';
+
+function useEditorFontsResolver() {
+	const [ loadedFontUrls, setLoadedFontUrls ] = useState( new Set() );
+
+	const { currentTheme = {}, fontFamilies = [] } = useSelect( ( select ) => {
+		return {
+			currentTheme:
+				// Disable Reason: Using 'core' as string to avoid circular dependency importing from @wordpress/core-data.
+				// eslint-disable-next-line @wordpress/data-no-store-string-literals
+				select( 'core' )?.getCurrentTheme(),
+			fontFamilies:
+				select( editorStore ).getSettings()?.__experimentalFeatures
+					?.typography?.fontFamilies,
+		};
+	}, [] );
+
+	const fontFaces = useMemo( () => {
+		return Object.values( fontFamilies )
+			.flat()
+			.map( ( family ) => family.fontFace )
+			.filter( Boolean )
+			.flat();
+	}, [ fontFamilies ] );
+
+	const loadFontFaceAsset = useCallback(
+		async ( fontFace, ownerDocument ) => {
+			if ( ! fontFace.src ) {
+				return;
+			}
+
+			const src = getDisplaySrcFromFontFace(
+				fontFace.src,
+				currentTheme?.stylesheet_uri
+			);
+
+			if ( ! src || loadedFontUrls.has( src ) ) {
+				return;
+			}
+
+			loadFontFaceInBrowser( fontFace, src, ownerDocument );
+			setLoadedFontUrls( ( prevUrls ) => new Set( prevUrls ).add( src ) );
+		},
+		[ currentTheme, loadedFontUrls ]
+	);
+
+	return useCallback(
+		( node ) => {
+			if ( ! node ) {
+				return;
+			}
+
+			const { ownerDocument } = node;
+			fontFaces.forEach( ( fontFace ) =>
+				loadFontFaceAsset( fontFace, ownerDocument )
+			);
+		},
+		[ fontFaces, loadFontFaceAsset ]
+	);
+}
+
+export default useEditorFontsResolver;

packages/block-editor/src/components/use-editor-fonts-resolver/utils.js

@@ -0,0 +1,103 @@
+/*
+ * Format the font face name to use in the font-family property of a font face.
+ *
+ * The input can be a string with the font face name or a string with multiple font face names separated by commas.
+ * It removes the leading and trailing quotes from the font face name.
+ *
+ * @param {string} input - The font face name.
+ * @return {string} The formatted font face name.
+ *
+ * Example:
+ * formatFontFaceName("Open Sans") => "Open Sans"
+ * formatFontFaceName("'Open Sans', sans-serif") => "Open Sans"
+ * formatFontFaceName(", 'Open Sans', 'Helvetica Neue', sans-serif") => "Open Sans"
+ */
+function formatFontFaceName( input ) {
+	if ( ! input ) {
+		return '';
+	}
+
+	let output = input.trim();
+	if ( output.includes( ',' ) ) {
+		output = output
+			.split( ',' )
+			// Finds the first item that is not an empty string.
+			.find( ( item ) => item.trim() !== '' )
+			.trim();
+	}
+	// Removes leading and trailing quotes.
+	output = output.replace( /^["']|["']$/g, '' );
+
+	// Firefox needs the font name to be wrapped in double quotes meanwhile other browsers don't.
+	if ( window.navigator.userAgent.toLowerCase().includes( 'firefox' ) ) {
+		output = `"${ output }"`;
+	}
+	return output;
+}
+
+/*
+ * Loads the font face from a URL and adds it to the browser.
+ * It also adds it to the iframe document.
+ */
+export async function loadFontFaceInBrowser( fontFace, source, documentRef ) {
+	if ( typeof source !== 'string' ) {
+		return;
+	}
+	const dataSource = `url(${ source })`;
+	const newFont = new window.FontFace(
+		formatFontFaceName( fontFace.fontFamily ),
+		dataSource,
+		{
+			style: fontFace.fontStyle,
+			weight: fontFace.fontWeight,
+		}
+	);
+
+	const loadedFace = await newFont.load();
+
+	// Add the font to the ref document.
+	documentRef.fonts.add( loadedFace );
+
+	// Add the font to the window document.
+	if ( documentRef !== window.document ) {
+		window.document.fonts.add( loadedFace );
+	}
+}
+
+function isUrlEncoded( url ) {
+	if ( typeof url !== 'string' ) {
+		return false;
+	}
+	return url !== decodeURIComponent( url );
+}
+
+/*
+ * Retrieves the display source from a font face src.
+ *
+ * @param {string|string[]} fontSrc - The font face src.
+ * @param {string} baseUrl - The base URL to resolve the src.
+ * @return {string|undefined} The display source or undefined if the input is invalid.
+ */
+export function getDisplaySrcFromFontFace( fontSrc, baseUrl ) {
+	if ( ! fontSrc ) {
+		return;
+	}
+
+	let src;
+	if ( Array.isArray( fontSrc ) ) {
+		src = fontSrc[ 0 ];
+	} else {
+		src = fontSrc;
+	}
+
+	if ( ! isUrlEncoded( src ) ) {
+		src = encodeURI( src );
+	}
+
+	// If baseUrl is provided, use it to resolve the src.
+	if ( src.startsWith( 'file:.' ) ) {
+		src = baseUrl + '/' + src.replace( 'file:./', '' );
+	}
+
+	return src;
+}