Font preload support in old Magento versions.

Plugin/PageConfigRenderer.php

@@ -0,0 +1,45 @@
+<?php
+
+namespace Swissup\Core\Plugin;
+
+class PageConfigRenderer
+{
+    /**
+     * Add font preload support in Magento < 2.3.3.
+     *
+     * Usage in `default_head_blocks.xml`:
+     *
+     *     <link rel="preload" src="fonts/muli.woff2"/>
+     *
+     * @param Magento\Framework\View\Page\Config\Renderer $subject
+     * @param string $result
+     * @return string
+     */
+    public function afterRenderAssets(
+        \Magento\Framework\View\Page\Config\Renderer $subject,
+        $result
+    ) {
+        if (strpos($result, 'rel="preload"') === false) {
+            return $result;
+        }
+
+        preg_match_all('/<link.*rel="preload".*\/>/', $result, $links);
+
+        foreach ($links[0] as $link) {
+            if (strpos($link, '.woff') === false ||
+                strpos($link, 'crossorigin="anonymous"') !== false
+            ) {
+                continue;
+            }
+
+            $newLink = str_replace(
+                'rel="preload"',
+                'rel="preload" as="font" crossorigin="anonymous"',
+                $link
+            );
+            $result = str_replace($link, $newLink, $result);
+        }
+
+        return $result;
+    }
+}

etc/frontend/di.xml

@@ -7,4 +7,12 @@
                 sortOrder="10"
                 disabled="false"/>
     </type>
+
+    <!-- font preload support in Magento < 2.3.3 -->
+    <type name="Magento\Framework\View\Page\Config\Renderer">
+        <plugin name="addMissingAssetAttributes"
+                type="Swissup\Core\Plugin\PageConfigRenderer"
+                sortOrder="10"
+                disabled="false"/>
+    </type>
 </config>