Speculation Rules | Clear prefetch & prerender cache after cart item action

Observer/AddClearSiteDataHeader.php

@@ -0,0 +1,32 @@
+<?php
+
+declare(strict_types=1);
+
+namespace MageOS\ThemeOptimization\Observer;
+
+use Magento\Framework\App\Response\HttpInterface;
+use Magento\Framework\Event\Observer;
+use Magento\Framework\Event\ObserverInterface;
+
+/**
+ * Stale speculative loads can be cleared using the `prefetchCache` and `prerenderCache` value of the `Clear-Site-Data` response header.
+ * See [MDN documentation](https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Clear-Site-Data#prefetchcache)
+ */
+class AddClearSiteDataHeader implements ObserverInterface
+{
+    public function execute(Observer $observer): void
+    {
+        /** @var HttpInterface|null $response */
+        $response = $observer->getResponse();
+
+        if (!$response) {
+            return;
+        }
+
+        $response->setHeader(
+            'Clear-Site-Data',
+            '"prefetchCache", "prerenderCache"',
+            true
+        );
+    }
+}

etc/events.xml

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+<?xml version="1.0"?>
+
+<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+        xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
+    <!-- Clear the prefetch & prerender cache after a product is successfully added to the cart -->
+    <event name="checkout_cart_add_product_complete">
+        <observer name="clear_site_data_header_add" instance="MageOS\ThemeOptimization\Observer\AddClearSiteDataHeader"/>
+    </event>
+    <!-- Clear the prefetch & prerender cache after a cart item is successfully updated -->
+    <event name="checkout_cart_update_item_complete">
+        <observer name="clear_site_data_header_update" instance="MageOS\ThemeOptimization\Observer\AddClearSiteDataHeader"/>
+    </event>
+</config>