Checkpoints - Use linked style sheets to control layout and presentation
CSS benefits accessibility primarily by separating document structure from presentation. Style sheets were designed to allow precise control - outside of markup - of character spacing, text alignment, object position on the page, font characteristics, etc. By separating style from markup, authors can simplify and clean up the markup in their content, making it more accessible at the same time.
In addition, advanced users can overwrite stylesheets with their own accommodations for a particular page. For example they might specify that all links are bold and red as this is a specifically helpful for them.
Accessibility (WCAG 2 Level A, 1.3.1)
The checkpoint examines the entire HTML source code of a page and reports if inline CSS is used on the page