@todo replace with `wp_trigger_error()`.
			_doing_it_wrong(
				__METHOD__,
				__( 'Font src must be a non-empty string or an array of strings.' ),
				'6.4.0'
			);
			return false;
		}

		// Validate the 'src' property.
		foreach ( (array) $font_face['src'] as $src ) {
			if ( empty( $src ) || ! is_string( $src ) ) {
				// @todo replace with `wp_trigger_error()`.
				_doing_it_wrong(
					__METHOD__,
					__( 'Each font src must be a non-empty string.' ),
					'6.4.0'
				);
				return false;
			}
		}

		// Check the font-weight.
		if ( ! is_string( $font_face['font-weight'] ) && ! is_int( $font_face['font-weight'] ) ) {
			// @todo replace with `wp_trigger_error()`.
			_doing_it_wrong(
				__METHOD__,
				__( 'Font font-weight must be a properly formatted string or integer.' ),
				'6.4.0'
			);
			return false;
		}

		// Check the font-display.
		if ( ! in_array( $font_face['font-display'], $this->valid_font_display, true ) ) {
			$font_face['font-display'] = $this->font_face_property_defaults['font-display'];
		}

		// Remove invalid properties.
		foreach ( $font_face as $property => $value ) {
			if ( ! in_array( $property, $this->valid_font_face_properties, true ) ) {
				unset( $font_face[ $property ] );
			}
		}

		return $font_face;
	}

	/**
	 * Gets the style element for wrapping the `@font-face` CSS.
	 *
	 * @since 6.4.0
	 *
	 * @return string The style element.
	 */
	private function get_style_element() {
		$attributes = $this->generate_style_element_attributes();

		return "<style id='wp-fonts-local'{$attributes}>\n%s\n</style>\n";
	}

	/**
	 * Gets the defined <style> element's attributes.
	 *
	 * @since 6.4.0
	 *
	 * @return string A string of attribute=value when defined, else, empty string.
	 */
	private function generate_style_element_attributes() {
		$attributes = '';
		foreach ( $this->style_tag_attrs as $name => $value ) {
			$attributes .= " {$name}='{$value}'";
		}
		return $attributes;
	}

	/**
	 * Gets the `@font-face` CSS styles for locally-hosted font files.
	 *
	 * This method does the following processing tasks:
	 *    1. Orchestrates an optimized `src` (with format) for browser support.
	 *    2. Generates the `@font-face` for all its fonts.
	 *
	 * @since 6.4.0
	 *
	 * @param array $font_faces The font-faces to generate @font-face CSS styles.
	 * @return string The `@font-face` CSS styles.
	 */
	private function get_css( $font_faces ) {
		$css = '';

		foreach ( $font_faces as $font_face ) {
				// Order the font's `src` items to optimize for browser support.
				$font_face = $this->order_src( $font_face );

				// Build the @font-face CSS for this font.
				$css .= '@font-face{' . $this->build_font_face_css( $font_face ) . '}' . "\n";
		}

		// Don't print the last newline character.
		return rtrim( $css, "\n" );
	}

	/**
	 * Orders `src` items to optimize for browser support.
	 *
	 * @since 6.4.0
	 *
	 * @param array $font_face Font face to process.
	 * @return array Font-face with ordered src items.
	 */
	private function order_src( array $font_face ) {
		if ( ! is_array( $font_face['src'] ) ) {
			$font_face['src'] = (array) $font_face['src'];
		}

		$src         = array();
		$src_ordered = array();

		foreach ( $font_face['src'] as $url ) {
			// Add data URIs first.
			if ( str_starts_with( trim( $url ), 'data:' ) ) {
				$src_ordered[] = array(
					'url'    => $url,
					'format' => 'data',
				);
				continue;
			}
			$format         = pathinfo( $url, PATHINFO_EXTENSION );
			$src[ $format ] = $url;
		}

		// Add woff2.
		if ( ! empty( $src['woff2'] ) ) {
			$src_ordered[] = array(
				'url'    => $src['woff2'],
				'format' => 'woff2',
			);
		}

		// Add woff.
		if ( ! empty( $src['woff'] ) ) {
			$src_ordered[] = array(
				'url'    => $src['woff'],
				'format' => 'woff',
			);
		}

		// Add ttf.
		if ( ! empty( $src['ttf'] ) ) {
			$src_ordered[] = array(
				'url'    => $src['ttf'],
				'format' => 'truetype',
			);
		}

		// Add eot.
		if ( ! empty( $src['eot'] ) ) {
			$src_ordered[] = array(
				'url'    => $src['eot'],
				'format' => 'embedded-opentype',
			);
		}

		// Add otf.
		if ( ! empty( $src['otf'] ) ) {
			$src_ordered[] = array(
				'url'    => $src['otf'],
				'format' => 'opentype',
			);
		}
		$font_face['src'] = $src_ordered;

		return $font_face;
	}

	/**
	 * Builds the font-family's CSS.
	 *
	 * @since 6.4.0
	 *
	 * @param array $font_face Font face to process.
	 * @return string This font-family's CSS.
	 */
	private function build_font_face_css( array $font_face ) {
		$css = '';

		/*
		 * Wrap font-family in quotes if it contains spaces
		 * and is not already wrapped in quotes.
		 */
		if (
			str_contains( $font_face['font-family'], ' ' ) &&
			! str_contains( $font_face['font-family'], '"' ) &&
			! str_contains( $font_face['font-family'], "'" )
		) {
			$font_face['font-family'] = '"' . $font_face['font-family'] . '"';
		}

		foreach ( $font_face as $key => $value ) {
			// Compile the "src" parameter.
			if ( 'src' === $key ) {
				$value = $this->compile_src( $value );
			}

			// If font-variation-settings is an array, convert it to a string.
			if ( 'font-variation-settings' === $key && is_array( $value ) ) {
				$value = $this->compile_variations( $value );
			}

			if ( ! empty( $value ) ) {
				$css .= "$key:$value;";
			}
		}

		return $css;
	}

	/**
	 * Compiles the `src` into valid CSS.
	 *
	 * @since 6.4.0
	 *
	 * @param array $value Value to process.
	 * @return string The CSS.
	 */
	private function compile_src( array $value ) {
		$src = '';

		foreach ( $value as $item ) {
			$src .= ( 'data' === $item['format'] )
				? ", url({$item['url']})"
				: ", url('{$item['url']}') format('{$item['format']}')";
		}

		$src = ltrim( $src, ', ' );
		return $src;
	}

	/**
	 * Compiles the font variation settings.
	 *
	 * @since 6.4.0
	 *
	 * @param array $font_variation_settings Array of font variation settings.
	 * @return string The CSS.
	 */
	private function compile_variations( array $font_variation_settings ) {
		$variations = '';

		foreach ( $font_variation_settings as $key => $value ) {
			$variations .= "$key $value";
		}

		return $variations;
	}
}