Discard invalid declarations when parsing CSS (#16093)

I discovered this when triaging an error someone had on Tailwind Play.

1. When we see a `;` we often assume a valid declaration precedes it but
that may not be the case
2. When we see the name of a custom property we assume everything that
follows will be a valid declaration but that is not necessarily the case
3. A bare identifier inside of a rule is treated as a declaration which
is not the case

This PR fixes all three of these by ignoring these invalid cases. Though
some should probably be turned into errors.

---------

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

CHANGELOG.md

@@ -19,6 +19,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Vite: Transform `<style>` blocks in HTML files ([#16069](https://github.com/tailwindlabs/tailwindcss/pull/16069))
 - Prevent camelCasing CSS custom properties added by JavaScript plugins ([#16103](https://github.com/tailwindlabs/tailwindcss/pull/16103))
 - Do not emit `@keyframes` in `@theme reference` ([#16120](https://github.com/tailwindlabs/tailwindcss/pull/16120))
+- Discard invalid declarations when parsing CSS ([#16093](https://github.com/tailwindlabs/tailwindcss/pull/16093))
 
 ## [4.0.1] - 2025-01-29
 

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

@@ -329,6 +329,28 @@ describe.each(['Unix', 'Windows'])('Line endings: %s', (lineEndings) => {
         ])
       })
 
+      it('should parse a custom property with an empty value', () => {
+        expect(parse('--foo:;')).toEqual([
+          {
+            kind: 'declaration',
+            property: '--foo',
+            value: '',
+            important: false,
+          },
+        ])
+      })
+
+      it('should parse a custom property with a space value', () => {
+        expect(parse('--foo: ;')).toEqual([
+          {
+            kind: 'declaration',
+            property: '--foo',
+            value: '',
+            important: false,
+          },
+        ])
+      })
+
       it('should parse a custom property with a block including nested "css"', () => {
         expect(
           parse(css`
@@ -1097,5 +1119,39 @@ describe.each(['Unix', 'Windows'])('Line endings: %s', (lineEndings) => {
         `),
       ).toThrowErrorMatchingInlineSnapshot(`[Error: Unterminated string: "Hello world!;"]`)
     })
+
+    it('should error when incomplete custom properties are used', () => {
+      expect(() => parse('--foo')).toThrowErrorMatchingInlineSnapshot(
+        `[Error: Invalid custom property, expected a value]`,
+      )
+    })
+
+    it('should error when incomplete custom properties are used inside rules', () => {
+      expect(() => parse('.foo { --bar }')).toThrowErrorMatchingInlineSnapshot(
+        `[Error: Invalid custom property, expected a value]`,
+      )
+    })
+
+    it('should error when a declaration is incomplete', () => {
+      expect(() => parse('.foo { bar }')).toThrowErrorMatchingInlineSnapshot(
+        `[Error: Invalid declaration: \`bar\`]`,
+      )
+    })
+
+    it('should error when a semicolon exists after an at-rule with a body', () => {
+      expect(() => parse('@plugin "foo" {} ;')).toThrowErrorMatchingInlineSnapshot(
+        `[Error: Unexpected semicolon]`,
+      )
+    })
+
+    it('should error when consecutive semicolons exist', () => {
+      expect(() => parse(';;;')).toThrowErrorMatchingInlineSnapshot(`[Error: Unexpected semicolon]`)
+    })
+
+    it('should error when consecutive semicolons exist after a declaration', () => {
+      expect(() => parse('.foo { color: red;;; }')).toThrowErrorMatchingInlineSnapshot(
+        `[Error: Unexpected semicolon]`,
+      )
+    })
   })
 })

packages/tailwindcss/src/css-parser.ts

@@ -286,6 +286,8 @@ export function parse(input: string) {
       }
 
       let declaration = parseDeclaration(buffer, colonIdx)
+      if (!declaration) throw new Error(`Invalid custom property, expected a value`)
+
       if (parent) {
         parent.nodes.push(declaration)
       } else {
@@ -337,6 +339,11 @@ export function parse(input: string) {
       closingBracketStack[closingBracketStack.length - 1] !== ')'
     ) {
       let declaration = parseDeclaration(buffer)
+      if (!declaration) {
+        if (buffer.length === 0) throw new Error('Unexpected semicolon')
+        throw new Error(`Invalid declaration: \`${buffer.trim()}\``)
+      }
+
       if (parent) {
         parent.nodes.push(declaration)
       } else {
@@ -435,7 +442,10 @@ export function parse(input: string) {
 
           // Attach the declaration to the parent.
           if (parent) {
-            parent.nodes.push(parseDeclaration(buffer, colonIdx))
+            let node = parseDeclaration(buffer, colonIdx)
+            if (!node) throw new Error(`Invalid declaration: \`${buffer.trim()}\``)
+
+            parent.nodes.push(node)
           }
         }
       }
@@ -543,7 +553,11 @@ export function parseAtRule(buffer: string, nodes: AstNode[] = []): AtRule {
   return atRule(buffer.trim(), '', nodes)
 }
 
-function parseDeclaration(buffer: string, colonIdx: number = buffer.indexOf(':')): Declaration {
+function parseDeclaration(
+  buffer: string,
+  colonIdx: number = buffer.indexOf(':'),
+): Declaration | null {
+  if (colonIdx === -1) return null
   let importantIdx = buffer.indexOf('!important', colonIdx + 1)
   return decl(
     buffer.slice(0, colonIdx).trim(),