Add noloading to all images in an AMP page

class-img-noloading-sanitizer.php

<?php
/**
 * Make sure that all <amp-img> elements have the noloading attribute.
 */
class Img_Noloading_Sanitizer extends AMP_Base_Sanitizer {
	public function sanitize() {
		foreach ( $this->dom->getElementsByTagName( 'amp-img' ) as $img ) {
			$img->setAttribute( 'noloading', '' );
		}
	}
}

img-noloading.php

<?php

add_filter( 'amp_content_sanitizers', function( $sanitizers ) {
	require_once __DIR__ . '/class-img-noloading-sanitizer.php';
	return array_merge(
		$sanitizers,
		array(
			'Img_Noloading_Sanitizer' => array(),
		)
	);
} );