The preload resource hint can give the browser a head start on loading your site's CSS and it also signals the browser to initiate an early fetch for an asset.
You can set it as an HTTP header in your server configuration: Link:
; rel=preload; as=style
Or as a tag in HTML like this: Link:
</css/styles.css>; rel=preload; as=style
Or as a <link> tag in HTML like this:
<link rel="preload" href="/css/styles.css" as="style">
First option is recommended, because this means that that it will initiate a server push event on most HTTP/2 implementations and that the browser will discover the hint earlier in the response headers, instead of later on in the response body.