functions.php

<?php
/**
 * Add skip-lazy class to first image in content.
 */
add_filter( 'the_content', function( $content ) {
	// Check if we have no content.
	if ( empty( $content ) ) {
		return $content;
	}

	// Check if content contains caption shortcode.
	if ( has_shortcode( $content, 'caption' ) ) {
		return $content;
	}

	if ( ! class_exists( '\Masterminds\HTML5' ) || ! class_exists( '\FlorianBrinkmann\LazyLoadResponsiveImages\Helpers' ) ) {
		return $content;
	}

	// Disable libxml errors.
	libxml_use_internal_errors( true );

	// Create new HTML5 object.
	$html5 = new \Masterminds\HTML5( array(
		'disable_html_ns' => true,
	) );

	// Preserve html entities, script tags and conditional IE comments.
	// @link https://github.com/ivopetkov/html5-dom-document-php.
	$content = preg_replace( '/&([a-zA-Z]*);/', 'lazy-loading-responsive-images-entity1-$1-end', $content );
	$content = preg_replace( '/&#([0-9]*);/', 'lazy-loading-responsive-images-entity2-$1-end', $content );
	$content = preg_replace( '/<!--\[([\w ]*)\]>/', '<!--[$1]>-->', $content );
	$content = str_replace( '<![endif]-->', '<!--<![endif]-->', $content );
	$content = str_replace( '<script>', '<!--<script>', $content );
	$content = str_replace( '<script ', '<!--<script ', $content );
	$content = str_replace( '</script>', '</script>-->', $content );

	// Load the HTML.
	$dom = $html5->loadHTML( $content );

	$xpath = new \DOMXPath( $dom );

	// Get all image nodes.
	// @link https://stackoverflow.com/a/19348287/7774451.
	$nodes = $xpath->query( "//img[not(ancestor-or-self::noscript)]" );

	$is_modified = false;

	$counter = 0;

	foreach ( $nodes as $node ) {
		$counter++;
		if ( $counter > 1 ) {
			break;
		}

		// Check if it is an element that should not be lazy loaded.
		// Get the classes as an array.
		$node_classes = explode( ' ', $node->getAttribute( 'class' ) );

		if ( in_array( 'skip-lazy', $node_classes ) ) {
			continue;
		}

		// Check if it is one of the supported elements and support for it is enabled.
		if ( 'img' === $node->tagName ) {
			// Get the classes.
			$classes = $node->getAttribute( 'class' );

			// Add lazyload class.
			$classes .= ' skip-lazy';

			// Set the class string.
			$node->setAttribute( 'class', $classes );
			$is_modified = true;
		}
	}

	if ( true === $is_modified ) {
		$helpers = new \FlorianBrinkmann\LazyLoadResponsiveImages\Helpers();
		$content = $helpers->save_html( $dom, $html5 );
	}

	// Restore the entities and script tags.
	// @link https://github.com/ivopetkov/html5-dom-document-php/blob/9560a96f63a7cf236aa18b4f2fbd5aab4d756f68/src/HTML5DOMDocument.php#L343.
	if ( strpos( $content, 'lazy-loading-responsive-images-entity') !== false || strpos( $content, '<!--<script' ) !== false ) {
		$content = preg_replace('/lazy-loading-responsive-images-entity1-(.*?)-end/', '&$1;', $content );
		$content = preg_replace('/lazy-loading-responsive-images-entity2-(.*?)-end/', '&#$1;', $content );
		$content = preg_replace( '/<!--\[([\w ]*)\]>-->/', '<!--[$1]>', $content );
		$content = str_replace( '<!--<![endif]-->', '<![endif]-->', $content );
		$content = str_replace( '<!--<script>', '<script>', $content );
		$content = str_replace( '<!--<script ', '<script ', $content );
		$content = str_replace( '</script>-->', '</script>', $content );
	}

	return $content;
}, 99 );