Why it matters
v4 shifted the scales: v3 shadow-sm is v4 shadow-xs, shadow → shadow-sm, rounded-sm → rounded-xs, blur-sm → blur-xs, ring is 1px instead of 3px, and outline-none became outline-hidden. Code pasted from v3 docs or registries renders one step off without any error.
How to fix it
When porting v3 code, rename to the v4 equivalents (or run npx @tailwindcss/upgrade). Only reported while v3 config signals remain in the project.
Example
// v3 // v4
shadow-sm rounded-sm ring → shadow-xs rounded-xs ring-3References
More lint rules
void lint reports 67 rules in this category. Static source checks with no browser: Tailwind v4 silent failures, Next.js 16 API traps, React render-body bugs, accessibility markup, SEO files and motion hygiene.
tw-unknown-classClass looks like a Tailwind utility but compiles to no CSStw-v3-arbitrary-var`utility-[--var]` is Tailwind v3 syntax; v4 needs `utility-(--var)`tw-js-config-ignored`tailwind.config.*` exists but the CSS entry has no `@config`tw-dark-variant-os`dark:` follows the OS, but the app toggles themes with a class/attributetw-important-soupMany `!important` utilities in one filetw-arbitrary-sprawlMany arbitrary colors / pixel values bypass the design tokenspage-level-use-client`"use client"` at the top of a page or layoutheavy-importHeavy dependency imported into client code