tailwind-v4

name	tailwind-v4
description	Tailwind CSS v4 usage guide and v3-to-v4 differences. This skill should be used when writing, reviewing, or refactoring any Tailwind CSS code in this repo. Triggers on tasks involving Tailwind classes, @theme blocks, CSS-first configuration, or cleanup of v3-era syntax.

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, ...)

CSS-first configuration

Tailwind v4 is configured in CSS, not JavaScript.

Use	Never use (v3-era)
@import 'tailwindcss'	@tailwind base/components/utilities
@theme { --color-x: ...; } for tokens that should generate utilities	tailwind.config.ts for new work
@utility name { ... } for custom utilities (works with variants)	@layer utilities { .name { ... } }
@custom-variant dark (&:is(.dark *))	JS plugins / addVariant
@source "path" / @source inline("...") for extra sources / safelisting	content array / safelist config
@variant dark { ... } to apply a Tailwind variant inside custom CSS	duplicating media queries / selectors
@reference "app.css" for @apply in scoped styles (Vue, CSS Modules)	duplicating stylesheet imports
var(--color-x) in CSS, getComputedStyle in JS	theme() function, resolveConfig
@config "…" / @plugin "…" only for existing JS-config integrations	adding new JS configs or plugins

@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.

v3 → v4 renames

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

v3	v4
shadow-sm / shadow	shadow-xs / shadow-sm
drop-shadow-sm / drop-shadow	drop-shadow-xs / drop-shadow-sm
blur-sm / blur	blur-xs / blur-sm
rounded-sm / rounded	rounded-xs / rounded-sm
outline-none	outline-hidden (a11y-safe); outline-none now truly removes it
ring (3px)	ring-3; the default ring is now 1px currentColor
bg-opacity-50, text-opacity-*	opacity modifier: bg-black/50, text-white/50
bg-gradient-to-r	bg-linear-to-r (plus new bg-conic-*, bg-radial-*)
!bg-red-500 (prefix)	bg-red-500! (suffix)
flex-shrink-* / flex-grow-*	shrink-* / grow-*
bg-[--var]	bg-(--var); brackets now require bg-[var(--var)]
grid-cols-[a,b] (commas)	underscores: grid-cols-[max-content_auto]

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:

Don't	Do
min-w-[400px], w-[600px]	min-w-100, w-150
h-[1.5rem] w-[1.5rem]	size-6
mt-[68px]	mt-17
grid-cols-[repeat(15,minmax(0,1fr))]	grid-cols-15
h-[100dvh], w-[100dvw], h-[1lh]	h-dvh, w-dvw, h-lh
max-w-[80rem]	max-w-7xl (container scale)
data-[current]:opacity-100	data-current:opacity-100 (values keep brackets: data-[state=open]:)
bg-[var(--row-bg)]	bg-(--row-bg); type hints when ambiguous: text-(color:--fg), text-(length:--size)

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:

Hand-written	Tailwind
.btn:active { ... } in CSS	active:bg-surface5
:focus-visible rules	focus-visible:outline-accent1
[aria-expanded="true"] selectors	aria-expanded:rotate-180 (any aria-* boolean or value)
[data-state="open"] selectors	data-[state=open]:opacity-100, boolean: data-current:
JS hover/focus state to style a sibling/child	group/group-hover:, peer/peer-checked:, in-focus:
parent-state selectors / JS "has child" checks	has-checked:bg-accent1, has-[>svg]:pl-8
mask-image / fade-out gradients in CSS	mask-b-from-80%, mask-t-from-*
@media (hover: none) blocks	pointer-coarse:/pointer-fine:
first/last/nth rules in CSS	first:, last:, odd:, nth-3:, not-first:
styling all children from CSS	*:rounded-full (children), **:data-avatar:rounded-full (descendants)

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-[...].