Don't treat \ as an escape inside CSS comments (#20508)

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## Summary

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Provide a summary of the issue and the changes you're making. How does
your change solve the problem?

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The CSS parser treated `\` inside comments as an escape and skipped the
character after it. Because of that, a comment ending in `\*/` was never
closed, and the CSS that followed was swallowed or turned into a broken
rule:

```css
/* C:\temp\*/
.a { color: red }
```

Before this change the `.a` rule disappeared entirely, and `/* \*/ .a {
color: red }` produced the selector `* \*/ .a`. The same thing happened
to comments inside declaration values, where the comment ran on until
the next `*/` it could find, pulling following declarations into the
value.

Per [CSS Syntax Level 3
§4.3.2](https://www.w3.org/TR/css-syntax-3/#consume-comment), a comment
ends at the first `*/` and escapes aren't processed inside comments.
This removes the backslash handling from both comment-scanning loops in
`css-parser.ts` (top-level and inside declaration values).

The existing test `/*Hello, \*\/ world!*/` keeps passing, since that
input contains no `*/` before the final one.

## Test plan

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Added two tests to `packages/tailwindcss/src/css-parser.test.ts` (both
run with Unix and Windows line endings):

- a top-level comment ending in `\*/` followed by a rule
- a comment ending in `\*/` inside a custom property value, followed by
another declaration

Both fail without the change to `css-parser.ts` and pass with it.

```sh
pnpm vitest run packages/tailwindcss/src/css-parser.test.ts
pnpm vitest run --project tailwindcss
pnpm run lint
```

---------

Co-authored-by: Robin Malfait <robin.malfait@shopify.com>
Co-authored-by: Robin Malfait <malfait.robin@gmail.com>

CHANGELOG.md

@@ -34,6 +34,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Don't warn about Angular's `::ng-deep` and `:host-context()` when optimizing CSS ([#20434](https://github.com/tailwindlabs/tailwindcss/pull/20434))
 - Don't generate CSS for candidates containing an empty additional modifier (e.g. `bg-red-500/50/` and `group-hover/foo//bar:flex`) ([#20466](https://github.com/tailwindlabs/tailwindcss/pull/20466))
 - Sort `min-*`, `max-*`, and container query variants with decimal values numerically (e.g. `min-[40.25rem]` before `min-[40.5rem]`) ([#20512](https://github.com/tailwindlabs/tailwindcss/pull/20512))
+- Ensure CSS comments ending with `\*/` are closed correctly instead of swallowing the CSS that follows (e.g. `/* C:\temp\*/`) ([#20508](https://github.com/tailwindlabs/tailwindcss/pull/20508))
 
 ## [4.3.3] - 2026-07-16
 

packages/tailwindcss/src/css-parser.test.ts

@@ -31,6 +31,23 @@ describe.each(['Unix', 'Windows'])('Line endings: %s', (lineEndings) => {
       ).toEqual([])
     })
 
+    it('should end a comment at `*/` even when it is preceded by a `\\`', () => {
+      expect(
+        parse(css`
+          /* C:\temp\*/
+          .foo {
+            color: red;
+          }
+        `),
+      ).toEqual([
+        {
+          kind: 'rule',
+          selector: '.foo',
+          nodes: [{ kind: 'declaration', property: 'color', value: 'red', important: false }],
+        },
+      ])
+    })
+
     it('should parse a comment inside of a selector and ignore it', () => {
       expect(
         parse(css`
@@ -448,6 +465,28 @@ describe.each(['Unix', 'Windows'])('Line endings: %s', (lineEndings) => {
         ])
       })
 
+      it('should end a comment in a custom property at `*/` even when it is preceded by a `\\`', () => {
+        expect(
+          parse(css`
+            --foo: /* C:\temp\*/ bar;
+            --bar: /* baz */ qux;
+          `),
+        ).toEqual([
+          {
+            kind: 'declaration',
+            property: '--foo',
+            value: '/* C:\\temp\\*/ bar',
+            important: false,
+          },
+          {
+            kind: 'declaration',
+            property: '--bar',
+            value: '/* baz */ qux',
+            important: false,
+          },
+        ])
+      })
+
       it('should parse empty custom properties', () => {
         expect(
           parse(css`

packages/tailwindcss/src/css-parser.ts

@@ -128,19 +128,24 @@ export function parse(input: string, opts?: ParseOptions) {
     //         ^^^^^^^^^^^^^
     // }
     // ```
+    //
+    // The escape character `\` in comments are ignored, this means that a
+    // the end of a comment preceded by `\` does _not_ mean that the end of the
+    // comment is escaped and therefore we have to keep parsing.
+    //
+    // This is a valid comment:
+    // ```
+    // /*C:\*/
+    // ```
+    // See: https://www.w3.org/TR/css-syntax-3/#consume-comment
     else if (currentChar === SLASH && input.charCodeAt(i + 1) === ASTERISK) {
       let start = i
 
       for (let j = i + 2; j < input.length; j++) {
         peekChar = input.charCodeAt(j)
 
-        // Current character is a `\` therefore the next character is escaped.
-        if (peekChar === BACKSLASH) {
-          j += 1
-        }
-
-        // End of the comment
-        else if (peekChar === ASTERISK && input.charCodeAt(j + 1) === SLASH) {
+        // End of the comment.
+        if (peekChar === ASTERISK && input.charCodeAt(j + 1) === SLASH) {
           i = j + 1
           break
         }
@@ -224,13 +229,9 @@ export function parse(input: string, opts?: ParseOptions) {
         else if (peekChar === SLASH && input.charCodeAt(j + 1) === ASTERISK) {
           for (let k = j + 2; k < input.length; k++) {
             peekChar = input.charCodeAt(k)
-            // Current character is a `\` therefore the next character is escaped.
-            if (peekChar === BACKSLASH) {
-              k += 1
-            }
 
-            // End of the comment
-            else if (peekChar === ASTERISK && input.charCodeAt(k + 1) === SLASH) {
+            // End of the comment.
+            if (peekChar === ASTERISK && input.charCodeAt(k + 1) === SLASH) {
               j = k + 1
               break
             }