Is it possible to add
"rel=preload" for all the core Magento CSS files?
I have added that for my custom CSS file.
how can this be added to all the core CSS files for Magento 2.4
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You can enable it via admin panel Stores > Settings > Configuration > ADVANCED > Developer or via terminal.
bin/magento config:set dev/css/use_css_critical_path 1
You can create a critical.css in your custom theme as the example below.
Reference: Magento 2.4 DevDocs - CSS critical path