Remove non-using styles from your CSS files.
1) Deleting old styles:
Supposing, you deleted a slider from the home page. To remove all styles associated with this slider, you need to pass the main page through the cleaner.
2) Splitting a large CSS file into several styles:
Supposing, you have one large CSS file. You need to divide the styles into 2 blocks.
For this, you need to do 2 checks for each set of pages. After each check, you will get a file with styles that are used in a certain set.
3) Segregation the pass-through styles:
Create an individual page with pass-through elements only: header, footer, H1 headers, etc. Run a check on this page.
The resulting styles are passed-through.