CSS shows up twice when Critical CSS Enabled

[brightonmike]

Preconditions (*)

1. 2.3.4

Steps to reproduce (*)

1. Enable use_css_critical_path

Expected result (*)

1. The stylesheets are only loaded once

Actual result (*)

1. styles-m.css, styles-l.css and print.css styles show up multiple times in developer tools.

I have traced the problem to the following method:
beforeSendResponse in module-theme/Controller/Result/AsyncCssPlugin.php

This line appears to be re-inserting the original style tags:

$content = str_replace('</body', implode("\n", $cssMatches) . "\n</body", $content);

If you comment out this line, the HTML is as expected, with each style element existing only once and preventing double download of stylesheets.

I think there's some confusion over the implementation of the following technique:

<link rel="preload" as="style" media="all" onload="this.onload=null;this.rel='stylesheet'"href="...en_US/css/styles-m.css">

I would expect that the purpose of the onload method is that rather than needing one <link> to preload and a separate <link> at the bottom of the HTML to then achieve the affect of asynchronously loading the CSS, this behaviour can be achieved with just one <link> element. After onload event has fired, the rel attribute swaps and applies the CSS.

[m2-assistant]

Hi @brightonmike. Thank you for your report.
To help us process this issue please make sure that you provided the following information:

• Summary of the issue
• Information on your environment
• Steps to reproduce
• Expected and actual results

Please make sure that the issue is reproducible on the vanilla Magento instance following Steps to reproduce. To deploy vanilla Magento instance on our environment, please, add a comment to the issue:

@magento give me 2.4-develop instance - upcoming 2.4.x release

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@brightonmike do you confirm that you were able to reproduce the issue on vanilla Magento instance following steps to reproduce?

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[brightonmike]

@magento give me 2.4-develop instance

[magento-engcom-team]

Hi @brightonmike. Thank you for your request. I'm working on Magento 2.4-develop instance for you

[magento-engcom-team]

Hi @brightonmike, here is your Magento instance.
Admin access: https://i-26498-2-4-develop.instances.magento-community.engineering/admin_95bd
Login: b4063ce2 Password: 32017648ea26
Instance will be terminated in up to 3 hours.

[brightonmike]

Please see screenshot. As you will see the styles are showing up twice per style sheet.

For myself personally, this is causing problems debugging FE code. It looks like this issue is to force the specificity of the stylesheets over the critical inlined CSS.

Couple of possible work arounds:

• Remove the onload event. The first style tags are simply for preload.
• Or, remove the second style tags and move the first style tags below the inlined CSS.

[m2-assistant]

Hi @engcom-Charlie. Thank you for working on this issue.
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[magento-engcom-team]

✅ Confirmed by @engcom-Charlie
Thank you for verifying the issue. Based on the provided information internal tickets MC-30771 were created

Issue Available: @engcom-Charlie, You will be automatically unassigned. Contributors/Maintainers can claim this issue to continue. To reclaim and continue work, reassign the ticket to yourself.