This is your reference when you need container queries, `calc-size()` for animating to auto heights, or OKLCH colors instead of HSL. It covers the modern stuff that actually shipped: view transitions, scroll-driven animations, anchor positioning, `text-wrap: balance`, and the new dynamic viewport units. Also handles newer HTML like customizable selects and popover attributes. Reach for this when refactoring old flexbox hacks or when you're starting fresh and don't want to write CSS like it's 2015. The cascade layers section alone will save you from specificity hell on larger projects.
npx skills add https://github.com/paulirish/dotfiles --skill modern-css