Show Lightning CSS warnings when optimizing/minifying in production (#18918)

This PR improves the DX by showing all the Lightning CSS warnings when
using a "production" build (or using `--optimize` or `--minify` flags
when using the CLI).

Right now Tailwind CSS itself doesn't care about the exact syntax you
are using in the CSS as long as it looks valid. We do this because
otherwise we would have to parse a lot more CSS syntax and validate it
even though it would be valid CSS in 99.99% of the cases.

Even worse, if you want to use newer CSS syntax that Tailwind CSS
doesn't validate yet, then you would get warnings for valid CSS.

Another reason why we don't do this is because the browser already does
a great job at ignoring invalid CSS.

So the linked issue #15872 would still silently fail in development
mode. In this case, everything would work, except the shadow with the
invalid syntax.

But in production mode, you would now get a proper warning from
Lightning CSS, because they try to optimize the CSS and remove invalid
CSS.

One potential issue here is that we run Lightning CSS on the generated
CSS, not on the input CSS. So the current output shows the warnings in
the output CSS not the input CSS. Any thoughts if we would just skip the
line numbers?

## Test plan

1. Everything works as before
2. In production mode, you would get warnings printed to the terminal.
This is done in `@tailwindcss/node` so the CLI/Vite/PostCSS plugins
would all get the same behavior.

Screenshots:

If you have a single issue:
<img width="977" height="441" alt="image"
src="https://github.com/user-attachments/assets/7b061ee9-b74f-4b40-aa05-cff67a21dfcc"
/>


If you have multiple issues:
<img width="2170" height="711" alt="image"
src="https://github.com/user-attachments/assets/a5bc9b0a-964b-465f-80f3-d30dd467e69c"
/>


Fixes: #15872

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"',
+    },
   },
 ])