speculative-loading-demo.php

<?php
/**
 * Plugin Name: Speculative Loading Eager Prerender Demo Plugin
 * Description: Overrides WordPress speculative loading rules to use "eager" and "prerender" for better perceived performance.
 * Version: 1.0
 * Author: Matthew Haines-Young
 */

// Hook into the filter WordPress uses to generate the speculationrules script.
add_filter('wp_speculation_rules', function($rules) {
    // Ensure the structure exists
    foreach (['prefetch', 'prerender'] as $type) {
        if (!empty($rules[$type]) && is_array($rules[$type])) {
            foreach ($rules[$type] as &$rule) {
                $rule['eagerness'] = 'eager';
                $rule['action'] = 'prerender'; // In case the browser supports 'action'
            }
            // If they were in 'prefetch', move to 'prerender'
            if ($type === 'prefetch') {
                $rules['prerender'] = array_merge($rules['prerender'] ?? [], $rules['prefetch']);
                unset($rules['prefetch']);
            }
        }
    }

    return $rules;
});