function wp_iframe_tag_add_loading_attr( $iframe, $context ) {
	// Iframes with fallback content (see `wp_filter_oembed_result()`) should not be lazy-loaded because they are
	// visually hidden initially.
	if ( false !== strpos( $iframe, ' data-secret="' ) ) {
		return $iframe;
	}

	// Iframes should have source and dimension attributes for the `loading` attribute to be added.
	if ( false === strpos( $iframe, ' src="' ) || false === strpos( $iframe, ' width="' ) || false === strpos( $iframe, ' height="' ) ) {
		return $iframe;
	}

	/**
	 * Filters the `loading` attribute value to add to an iframe. Default `lazy`.
	 *
	 * Returning `false` or an empty string will not add the attribute.
	 * Returning `true` will add the default value.
	 *
	 * @since 5.7.0
	 *
	 * @param string|bool $value   The `loading` attribute value. Returning a falsey value will result in
	 *                             the attribute being omitted for the iframe. Default 'lazy'.
	 * @param string      $iframe  The HTML `iframe` tag to be filtered.
	 * @param string      $context Additional context about how the function was called or where the iframe tag is.
	 */
	$value = apply_filters( 'wp_iframe_tag_add_loading_attr', 'lazy', $iframe, $context );

	if ( $value ) {
		if ( ! in_array( $value, array( 'lazy', 'eager' ), true ) ) {
			$value = 'lazy';
		}

		return str_replace( '<iframe', '<iframe loading="' . esc_attr( $value ) . '"', $iframe );
	}

	return $iframe;
}