I'm declaring preload links for a CSS file containg ONLY @font-face
rules.
Should I add <link rel="preload" href="styles.css" as="style">
as="style" or <link rel="preload" href="styles.css" as="font">
as="font"?
Since you are attempting to load styles.css
which is a style sheet, you should use as="style"
If you were loading a font it would look something like this <link rel="preload" href="fonts/some-webfont.woff2" as="font" type="font/woff2" crossorigin="anonymous">
If you have a look at this article https://developer.mozilla.org/en-US/docs/Web/HTML/Preloading_content you can see that there are a number of different types of content that can be pre-loaded using this technique.