I need to Defer merged css using JavaScript. Is there any possibilities to defer merged css. Example code i want to do.

    <noscript id="deferred-styles">
      <link rel="stylesheet" type="text/css" href="https://xxxx.xx/static/version1xxxx/_cache/merged/f64d21109ccdd1dc8924b35f2614e40f.min.css"/>

  var loadDeferredStyles = function() {
    var addStylesNode = document.getElementById("deferred-styles");
    var replacement = document.createElement("div");
    replacement.innerHTML = addStylesNode.textContent;
  var raf = window.requestAnimationFrame || window.mozRequestAnimationFrame ||
      window.webkitRequestAnimationFrame || window.msRequestAnimationFrame;
  if (raf) raf(function() { window.setTimeout(loadDeferredStyles, 0); });
  else window.addEventListener('load', loadDeferredStyles);
  • Is there no one can do this? Commented Oct 13, 2020 at 7:42

1 Answer 1


I have a method that works but I'm not 100% confident I'm using best practices. The functions that merge and add CSS to the site header are in the Magento Framework module:


The functions you need to edit are protected so we have to use preferences instead of say, a plugin. Make a new module in app/code/[Vendor]/[Namespace] with the usual files:




<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
    <module name="[Vendor]_[Namespace]" setup_version="1.0.0">
            <module name="Magento_Framework" />


<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../lib/internal/Magento/Framework/ObjectManager/etc/config.xsd">
    <preference for="Magento\Framework\View\Page\Config\Renderer" type="[Vendor]\[Namespace]\View\Page\Config\Renderer" />

Last thing is to make this file and copy all the Magento code in to it:


copy the code from:


You'll need to update the namespace and class definitions:

namespace [Vendor]\[Namespace]\View\Page\Config;
class Renderer implements \Magento\Framework\View\Page\Config\RendererInterface

And then you can get to customising the functions inside. I followed the Google page speed guide on deferring CSS by altering these two functions as follows:

 * Add default attributes
 * @param string $contentType
 * @param string $attributes
 * @return string
protected function addDefaultAttributes($contentType, $attributes)
    if ($contentType === 'js') {
        return ' type="text/javascript" ' . $attributes;

    if ($contentType === 'css') {
        return ' rel="preload" type="text/css" as="style" onload="this.onload=null;this.rel=\'stylesheet\'" ' . ($attributes ?: ' media="all"');

    if ($this->canTypeBeFont($contentType)) {
        return 'rel="preload" as="font" crossorigin="anonymous"';

    return $attributes;


 * Render HTML tags referencing corresponding URLs
 * @param \Magento\Framework\View\Asset\PropertyGroup $group
 * @return string
protected function renderAssetHtml(\Magento\Framework\View\Asset\PropertyGroup $group)
    $assets = $this->processMerge($group->getAll(), $group);
    $attributes = $this->getGroupAttributes($group);

    $result = '';
    $template = '';
    try {
        /** @var $asset \Magento\Framework\View\Asset\AssetInterface */
        foreach ($assets as $asset) {
            $template = $this->getAssetTemplate(
                $this->addDefaultAttributes($this->getAssetContentType($asset), $attributes)
            if ($group->getProperty(GroupedCollection::PROPERTY_CONTENT_TYPE) === 'css') {
                $result .= sprintf($template, $asset->getUrl(), $asset->getUrl());
            } else {
                $result .= sprintf($template, $asset->getUrl());
    } catch (LocalizedException $e) {
        $result .= sprintf($template, $this->urlBuilder->getUrl('', ['_direct' => 'core/index/notFound']));
    return $result;

It seemed to satisfy the Lighthouse tool but it's not tested thoroughly. Good luck with this!

EDIT: This change affects the backend CSS loading as well... not ideal but not website breaking.

  • When I tested this I got: Error: Class '****\DeferCss\View\Page\Config\Generator\Head' not found in /app/code/****/DeferCss/View/Page/Config/Renderer.php: Commented Aug 3, 2021 at 21:20
  • You might need to run setup:upgrade (and di:compile in production) Commented Aug 5, 2021 at 8:37
  • Tried that no joy sadly Commented Aug 5, 2021 at 18:20

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