Show Lightning CSS warnings when optimizing/minifying in production

CHANGELOG.md

@@ -21,6 +21,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Do not generate `grid-row` utilities when configuring `grid-row-start` or `grid-row-end` ([#18907](https://github.com/tailwindlabs/tailwindcss/pull/18907))
 - Prevent duplicate CSS when overwriting a static utility with a theme key ([#18056](https://github.com/tailwindlabs/tailwindcss/pull/18056))
 - Do not migrate `variant = 'outline'` during upgrades ([#18922](https://github.com/tailwindlabs/tailwindcss/pull/18922))
+- Show Lightning CSS warnings (if any) when optimizing/minifying ([#18918](https://github.com/tailwindlabs/tailwindcss/pull/18918))
 
 ## [4.1.13] - 2025-09-03
 

packages/@tailwindcss-node/src/optimize.ts

@@ -60,6 +60,48 @@ export function optimize(
   let result = optimize(Buffer.from(input), map)
   map = result.map?.toString()
 
+  // Because of `errorRecovery: true`, there could be warnings, so let's let the
+  // user know about them.
+  if (process.env.NODE_ENV !== 'test' && result.warnings.length > 0) {
+    let lines = input.split('\n')
+
+    let output = [
+      `Found ${result.warnings.length} ${result.warnings.length === 1 ? 'warning' : 'warnings'} while optimizing generated CSS:`,
+    ]
+
+    for (let [idx, warning] of result.warnings.entries()) {
+      output.push('')
+      if (result.warnings.length > 1) {
+        output.push(`Issue #${idx + 1}:`)
+      }
+
+      let context = 2
+
+      let start = Math.max(0, warning.loc.line - context - 1)
+      let end = Math.min(lines.length, warning.loc.line + context)
+
+      let snippet = lines.slice(start, end).map((line, idx) => {
+        if (start + idx + 1 === warning.loc.line) {
+          return `${dim(`\u2502`)} ${line}`
+        } else {
+          return dim(`\u2502 ${line}`)
+        }
+      })
+
+      snippet.splice(
+        warning.loc.line - start,
+        0,
+        `${dim('\u2506')}${' '.repeat(warning.loc.column - 1)} ${yellow(`${dim('^--')} ${warning.message}`)}`,
+        `${dim('\u2506')}`,
+      )
+
+      output.push(...snippet)
+    }
+    output.push('')
+
+    console.warn(output.join('\n'))
+  }
+
   result = optimize(result.code, map)
   map = result.map?.toString()
 
@@ -88,3 +130,11 @@ export function optimize(
     map,
   }
 }
+
+function dim(str: string) {
+  return `\x1B[2m${str}\x1B[22m`
+}
+
+function yellow(str: string) {
+  return `\x1B[33m${str}\x1B[39m`
+}

packages/@tailwindcss-node/tsup.config.ts

@@ -6,23 +6,35 @@ export default defineConfig([
     minify: true,
     dts: true,
     entry: ['src/index.cts'],
+    define: {
+      'process.env.NODE_ENV': '"production"',
+    },
   },
   {
     format: ['esm'],
     minify: true,
     dts: true,
     entry: ['src/index.ts'],
+    define: {
+      'process.env.NODE_ENV': '"production"',
+    },
   },
   {
     format: ['esm'],
     minify: true,
     dts: true,
     entry: ['src/esm-cache.loader.mts'],
+    define: {
+      'process.env.NODE_ENV': '"production"',
+    },
   },
   {
     format: ['cjs'],
     minify: true,
     dts: true,
     entry: ['src/require-cache.cts'],
+    define: {
+      'process.env.NODE_ENV': '"production"',
+    },
   },
 ])