Tailwind CSS v4

How to write idiomatic Tailwind v4 and spot v3-era syntax that still compiles but should not appear in new code.

Version and sources

Check the pinned version before using recent utilities: the playground packages pin tailwindcss in their package.json (4.2.2 at the time of writing — v4.2 features like logical properties and font-features-* are available; v4.3 additions like scrollbar-*, zoom-*, tab-* are not). When unsure whether a utility, variant, or directive exists in the pinned version, verify against the docs instead of guessing:

• Utility/variant reference: https://tailwindcss.com/docs
• v3 → v4 migration: https://tailwindcss.com/docs/upgrade-guide
• What each minor added: https://tailwindcss.com/blog/tailwindcss-v4 (and /tailwindcss-v4-1, /tailwindcss-v4-3, ...)

v3 → v4 renames

Bare names shifted one step down the scale, so the v3 spelling silently renders smaller or lighter:

Prefer generated utilities over arbitrary values

The spacing scale is infinite — every number compiles via calc(var(--spacing) * n) — so most v3-era arbitrary values have a named form:

Square brackets remain correct for true one-offs: max-h-[calc(100dvh-3rem)], grid-cols-[200px_minmax(0,1fr)], and arbitrary properties like [mask-type:luminance].

Class strings must stay complete and statically detectable: map props to full strings ({ success: 'bg-positive1' }[tone]), never build fragments like `bg-${tone}-500`.

Don't hand-write CSS or JS for what a variant covers

Before writing a stylesheet rule, a style prop, or an event handler for styling, check for a variant:

New capabilities — reach for these before hacks or JS

• field-sizing-content — auto-growing textarea without a JS resize listener.
• wrap-anywhere / wrap-break-word — long-word breaking inside flex without the min-w-0 hack.
• items-center-safe, justify-center-safe — centering that falls back to start on overflow.
• pointer-coarse: / pointer-fine: — adapt touch targets without user-agent sniffing.
• user-valid: / user-invalid: — validation styling only after user interaction (unlike :valid).
• starting: (+ transition-discrete for display/popover) — enter transitions without JS mount tricks.
• text-shadow-*, mask-t-from-*/mask-b-to-* (fade-out edges), scheme-dark (native controls/scrollbars), 3D transforms (rotate-x-*, perspective-*).

Behavior changes to remember

• Transforms use individual CSS properties: custom transition lists need transition-[opacity,scale], not transition-[opacity,transform]; reset with scale-none/rotate-none/translate-none.
• hover: only applies on hover-capable devices (@media (hover: hover)) — never gate required touch functionality behind hover.
• Default border-*, divide-*, ring, and outline colors are currentColor — set an explicit color when the color matters.
• Variant stacking applies left to right: v3 first:*:pt-0 is now *:first:pt-0.
• space-x/y-* changed selectors (:not(:last-child)); prefer flex/grid with gap-*.
• Container queries are built in: @container on the parent, @sm:/@max-md: on children.
• Useful v4 variants: starting: (enter transitions), not-*, in-* (like group-* without the group class), nth-*, *: (direct children), **: (descendants), inert:.

Motion

• Gate decorative animation: motion-safe:animate-spin motion-reduce:animate-none.
• Reusable animations are --animate-* tokens with their @keyframes inside @theme; local one-offs use animate-(--local-animation) or animate-[...].

npx skills add https://github.com/mastra-ai/mastra --skill tailwind-v4

CSS-first configuration

Tailwind v4 is configured in CSS, not JavaScript.

@theme variables are API: each one emits a native CSS variable AND generates utilities (--color-* → bg-*/text-*/border-*/..., --text-* → text-*, --shadow-* → shadow-*, --animate-* → animate-*, --breakpoint-* → responsive variants). A plain :root { --x: ...; } variable generates nothing — use it for runtime-only values. When a token's value references another variable (--color-x: var(--y)), declare it in @theme inline so the utility resolves the reference at the declaration site. In custom CSS, --alpha(var(--color-x) / 50%) and --spacing(4) replace v3 theme() math.