Preload WordPress theme.json custom fonts

functions.php

<?php

/**
 * Preload fonts.
 *
 * @return void
 */
function preload_fonts() {
	$fonts = \WP_Font_Face_Resolver::get_fonts_from_theme_json();

	if ( empty( $fonts ) ) {
		return;
	}

	foreach ( $fonts as $font ) {
		if ( ! is_array( $font ) || empty( $font ) ) {
			continue;
		}

		foreach ( $font as $variation ) {
			$src  = $variation['src'] ?? array();
			$type = null;

			if ( empty( $src ) ) {
				continue;
			}

			if ( ! is_array( $src ) ) {
				$src = array( $src );
			}

			foreach ( $src as $uri ) {
				$extension = pathinfo( $uri, PATHINFO_EXTENSION );

				if ( ! $extension ) {
					continue;
				}

				printf(
					'<link rel="preload" href="%s" as="font" type="font/%s" crossorigin="anonymous">',
					esc_url( $uri ),
					esc_attr( $extension )
				);
			}
		}
	}
}
add_action( 'wp_head', __NAMESPACE__ . '\preload_fonts' );