See https://github.com/WordPress/performance/pull/1471#issuecomment-2289389108

auto-sizes-via-html-tag-processor.php

<?php
/**
 * Plugin Name: Auto-Sizes via HTML Tag Processor
 */

function auto_sizes_update_content_img_tag_with_html_tag_processor( $html ): string {
	if ( ! is_string( $html ) ) {
		$html = '';
	}

	$processor = new WP_HTML_Tag_Processor( $html );

	// Bail if there is no IMG tag.
	if ( ! $processor->next_tag( array( 'tag_name' => 'IMG' ) ) ) {
		return $html;
	}

	// Bail early if the image is not lazy-loaded.
	if ( 'lazy' !== $processor->get_attribute( 'loading' ) ) {
		return $html;
	}

	$sizes = $processor->get_attribute( 'sizes' );

	// Bail early if the image is not responsive.
	if ( ! is_string( $sizes ) ) {
		return $html;
	}

	// Don't add 'auto' to the sizes attribute if it already exists.
	if ( auto_sizes_attribute_includes_valid_auto( $sizes ) ) {
		return $html;
	}

	$processor->set_attribute( 'sizes', "AUTO, $sizes" ); // Upper-case AUTO just to make it clear when this version is running.

	return $processor->get_updated_html();
}

add_action(
	'plugins_loaded',
	static function () {
		remove_filter( 'wp_content_img_tag', 'auto_sizes_update_content_img_tag' );
		if ( isset( $_GET['auto_sizes_via_html_tag_processor_disabled'] ) ) {
			$function = 'auto_sizes_update_content_img_tag';
		} else {
			$function = 'auto_sizes_update_content_img_tag_with_html_tag_processor';
		}
		// TODO: benchmark-server-timing doesn't seem to be computing the median correctly here. I'm seeing it output identical values across the columns for multiple URLs passed via -f.
		//if ( function_exists( 'perflab_wrap_server_timing' ) ) {
		//	$function = perflab_wrap_server_timing( $function, 'auto_sizes_update_content_img_tag', 'exist' );
		//}
		add_filter( 'wp_content_img_tag', $function );
	}
);