CSS

Tailwind CSS v4 is an architectural upgrade: goodbye tailwind.config.js, declare design tokens in @theme blocks inside native CSS files. Single @import "tailwindcss" entry replaces postcss.config; built-in CSS vars, native container queries, oxc engine 10-40x faster, simplified color functions.

Practical Google Core Web Vitals playbook: LCP via preconnects/image-priority/CDN, CLS elimination via image dimensions/font-loading/placeholders, INP via long-task splitting + debouncing.

Modern responsive CSS pillars: Container Queries for component-level responsiveness based on parent size (not viewport), plus :has() parent selector for ancestor styling based on descendants.