https://google.com

Google's homepage is famous for its minimalism, but the inline stylesheet that powers it is dense and aggressive on selector specificity. Plenty of ID selectors are used to keep search-box logic isolated.

https://twitter.com

Twitter's CSS is dominated by component-level utility classes auto-generated by their build pipeline. Class counts dwarf selectors of any other type.

https://github.com

GitHub's Primer-based design system shows up clearly in the stats: predictable selectors, a calm color palette, and a sane number of font sizes for an app of its size.

https://apple.com

Apple's marketing pages emphasize big typography over reusable components, leading to many unique font sizes and a wider color set than you might expect.

https://facebook.com

Facebook leans heavily on atomic CSS produced by their internal build, resulting in tens of thousands of single-property classes and very low cohesion per selector.

https://tumblr.com

Tumblr blends a curated marketing surface with user-generated themes, giving it a more eclectic CSS profile and a lot of media-specific selectors.