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function wp_iframe_tag_add_loading_attr( $iframe, $context ) {
	/*
	 * Get loading attribute value to use. This must occur before the conditional check below so that even iframes that
	 * are ineligible for being lazy-loaded are considered.
	 */
	$optimization_attrs = wp_get_loading_optimization_attributes(
		'iframe',
		array(
			/*
			 * The concrete values for width and height are not important here for now
			 * since fetchpriority is not yet supported for iframes.
			 * TODO: Use WP_HTML_Tag_Processor to extract actual values once support is
			 * added.
			 */
			'width'   => str_contains( $iframe, ' width="' ) ? 100 : null,
			'height'  => str_contains( $iframe, ' height="' ) ? 100 : null,
			// This function is never called when a 'loading' attribute is already present.
			'loading' => null,
		),
		$context
	);

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

	$value = isset( $optimization_attrs['loading'] ) ? $optimization_attrs['loading'] : false;

	/**
	 * 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.
	 * @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', $value, $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;
}