Описание тега critical-css
Critical CSS is a paid tool that optimizes page loading by making sure 'top of the fold' CSS renders before or while the HTML itself appears on the page.
Critical CSS ( https://criticalcss.com/) is a paid tool that prioritizes site rendering by optimizing and reducing the number and size of CSS files that need to be parsed by the browser before the styles load.
It includes WordPress integration, and there's also an API available for an additional price, to help generate above-the-fold optimized CSS files.