Critical CSS in WordPress: what it is, how to generate it and apply it by template
This is where critical CSS comes in: a very simple technique to understand (though not always easy to implement correctly), which consists of extracting the minimum CSS necessary to render the content "above the fold" and serving it inline (embedded directly in the HTML) in the `<head>` tag. The rest of the CSS is loaded later, preventing it from blocking the initial render.
The idea behind this article isn't to give you a "one-size-fits-all solution," because every WordPress stack (combination of technologies) is unique (theme, plugins, caching, CDN). The idea is that you leave with a process: detecting what's blocking, generating the correct critical CSS, applying it context-sensitively, and maintaining it without it becoming a maintenance nightmare.
If you're looking to improve Core Web Vitals with changes that have a real impact, critical CSS is often one of the best returns when your website is still loading too much CSS before rendering.