Tailwind CSS

Content Configuration

• content array in tailwind.config.js must include ALL files with classes—missing paths = missing styles in production
• Glob patterns: "./src/**/*.{js,jsx,ts,tsx,html}" covers nested directories
• Dynamic class names like bg-${color}-500 won't be detected—use complete class names or safelist
• Check production build size—if unexpectedly small, content paths are wrong

Responsive Prefixes

• Mobile-first: unprefixed styles apply to all sizes, md: applies at medium AND above
• sm:hidden md:block means hidden on small, visible on medium+—not "only on medium"
• Breakpoints: sm(640px), md(768px), lg(1024px), xl(1280px), 2xl(1536px)
• Custom breakpoints in config override defaults—use extend.screens to add without replacing

Dark Mode

• dark: prefix requires darkMode: 'class' in config—won't work with default media strategy if you need manual toggle
• Dark class on <html> or <body>, not on individual components
• dark:bg-gray-900 only applies when ancestor has class="dark"
• System preference: darkMode: 'media' uses prefers-color-scheme

State Variants

• hover:, focus:, active: work as expected
• group-hover: requires group class on parent—child reacts to parent hover
• peer-focus: requires peer class on sibling AND sibling must come first in DOM
• Stack variants: dark:hover:bg-gray-700 applies on hover in dark mode

Arbitrary Values

• bg-[#1da1f2] for one-off colors—brackets for any arbitrary value
• w-[calc(100%-2rem)] for calc expressions
• grid-cols-[1fr_2fr_1fr] underscores for spaces in values
• Arbitrary properties: [mask-type:alpha] for unsupported CSS properties

@apply Traps

• @apply in component CSS loses responsive/state variants—@apply hover:bg-blue-500 doesn't work as expected
• Order in @apply matters unlike HTML classes—later utilities override earlier
• Prefer HTML classes over @apply—easier to maintain, better tree-shaking
• If you must use @apply, keep it simple: base styles only

Configuration

• extend adds to defaults: extend: { colors: { brand: '#xxx' } } keeps all existing colors
• Top-level replaces defaults: colors: { brand: '#xxx' } removes all default colors
• theme() function in CSS: border-color: theme('colors.gray.200')
• Plugin order matters—later plugins can override earlier ones

Important Modifier

• ! prefix forces important: !mt-4 generates margin-top: 1rem !important
• Use sparingly—usually indicates specificity battle that should be fixed
• important: true in config makes ALL utilities important—avoid, breaks third-party CSS
• important: '#app' scopes specificity to selector—better than global important

Common Mistakes

• class="px-4 px-6" last one wins in stylesheet, not in HTML—both get applied, cascade decides
• Forgetting overflow-hidden with rounded-* on parent with absolute children
• h-screen doesn't account for mobile browser chrome—use h-dvh (dynamic viewport height)
• truncate needs width constraint or max-w-* to actually truncate

Performance

• JIT is default since v3—generates only used classes, no purge needed
• Avoid safelist with patterns like bg-*—defeats tree-shaking
• @layer components for reusable component styles—proper cascade order
• Large arbitrary values generate unique classes—extract to config if repeated