Render Blocking CSS. By Ilya Grigorik. By default, CSS is treated as a render blocking resource, which means that the browser won’t render any processed content until the CSSOM is constructed. Make sure to keep your CSS lean, deliver it as quickly as possible, and use media types and queries to unblock rendering.

https://developers.google.com/web/fundamentals/performance/critical-rendering-path/render-blocking-css

Read more about removing render blocking CSS:

How to Defer Render Blocking CSS

How to Defer Render Blocking CSS