Swift Performance can optimize automatically CSS delivery. For this Swift Performance will find and merge all embedded or inline CSS on the page, and merge them into one CSS file.
In this merged CSS Swift Performance can easily manipulate things (shorten URLs, color codes, force font-display:swap etc), and also can generate critical CSS from it.
That because it is always recommended to use Merge Styles feature and enable Critical CSS and Minify CSS.
In rare cases, if optimization cause minor differences, you can fix them with Extra CSS.
Disable Full CSS
If you choose Unused CSS mode for Critical CSS you can disable the full CSS completely. Usually you will need to add some Extra Critical CSS to get the best result.
Load Full CSS On Scroll
If you choose Viewport based Critical CSS, you can load the full CSS only when the user start scrolling. It is always recommended to use this feature.
Print critical CSS inline
It can speed up the rendering process, however you can’t take advantage of the browser caching. It is recommended to use if you have a one-page site, or only a few subsites.
Print full CSS inline
Unlike Print critical CSS inline, Print full CSS inline in most cases is not recommended to use. This is a special feature only for special cases, when WordPress can’t write files on the server.
Swift Performance will generate global CSS files – if it is possible – by default, which is suitable for most sites. See when you should use Separate Styles
Bypass CSS Import
It is recommended to use. If you enable this feature, Swift Performance will find CSS imports, and add them to the merged CSS instead importing them. Less DNS lookup (if it is a 3rd party file), and less HTTP requests.
You can exclude files from merging/optimization one by one with Exclude Styles option. You can use full or partial URLs/filenames here.
You can also Exclude Inline Styles if it is necessary. To exclude an inline style you need to add some unique string from it.
With Exclude 3rd Party CSS you can exclude easily all 3rd party CSS. It is not recommended, as usually 3rd party CSS can be further optimized, however sometimes you have to: merging 3rd party CSS can use more CPU, and some service like Google Fonts sometimes blocks these requests.