Do not emit empty rules/at-rules (#16121)

This PR is an optimization where it will not emit empty rules and
at-rules. I noticed this while working on
#16120 where we emitted:
```css
:root, :host {
}
```

There are some exceptions for "empty" at-rules, such as:

```css
@charset "UTF-8";
@layer foo, bar, baz;
@Custom-Media --modern (color), (hover);
@namespace "http://www.w3.org/1999/xhtml";
```

These don't have a body, but they still have a purpose and therefore
they will be emitted.

However, if you look at this:

```css
/* Empty rule */
.foo {
}

/* Empty rule, with nesting */
.foo {
  .bar {
  }
  .baz {
  }
}

/* Empty rule, with special case '&' rules */
.foo {
  & {
    &:hover {
    }
    &:focus {
    }
  }
}

/* Empty at-rule */
@media (min-width: 768px) {
}

/* Empty at-rule with nesting*/
@media (min-width: 768px) {
  .foo {
  }

  @media (min-width: 1024px) {
    .bar {
    }
  }
}
```

None of these will be emitted.

---------

Co-authored-by: Jordan Pittman <jordan@cryptica.me>

CHANGELOG.md

@@ -20,6 +20,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - 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))
+- Do not emit empty CSS rules and at-rules ([#16121](https://github.com/tailwindlabs/tailwindcss/pull/16121))
 
 ## [4.0.1] - 2025-01-29
 

integrations/cli/index.test.ts

@@ -368,12 +368,7 @@ describe.each([
         `,
       )
 
-      await fs.expectFileToContain('project-a/dist/out.css', [
-        css`
-          :root, :host {
-          }
-        `,
-      ])
+      await fs.expectFileToContain('project-a/dist/out.css', [css``])
     },
   )
 

packages/tailwindcss/src/ast.test.ts

@@ -2,6 +2,8 @@ import { expect, it } from 'vitest'
 import { context, decl, optimizeAst, styleRule, toCss, walk, WalkAction } from './ast'
 import * as CSS from './css-parser'
 
+const css = String.raw
+
 it('should pretty print an AST', () => {
   expect(toCss(optimizeAst(CSS.parse('.foo{color:red;&:hover{color:blue;}}'))))
     .toMatchInlineSnapshot(`
@@ -95,3 +97,91 @@ it('should stop walking when returning `WalkAction.Stop`', () => {
     }
   `)
 })
+
+it('should not emit empty rules once optimized', () => {
+  let ast = CSS.parse(css`
+    /* Empty rule */
+    .foo {
+    }
+
+    /* Empty rule, with nesting */
+    .foo {
+      .bar {
+      }
+      .baz {
+      }
+    }
+
+    /* Empty rule, with special case '&' rules */
+    .foo {
+      & {
+        &:hover {
+        }
+        &:focus {
+        }
+      }
+    }
+
+    /* Empty at-rule */
+    @media (min-width: 768px) {
+    }
+
+    /* Empty at-rule with nesting*/
+    @media (min-width: 768px) {
+      .foo {
+      }
+
+      @media (min-width: 1024px) {
+        .bar {
+        }
+      }
+    }
+
+    /* Exceptions: */
+    @charset "UTF-8";
+    @layer foo, bar, baz;
+    @custom-media --modern (color), (hover);
+    @namespace 'http://www.w3.org/1999/xhtml';
+  `)
+
+  expect(toCss(ast)).toMatchInlineSnapshot(`
+    ".foo {
+    }
+    .foo {
+      .bar {
+      }
+      .baz {
+      }
+    }
+    .foo {
+      & {
+        &:hover {
+        }
+        &:focus {
+        }
+      }
+    }
+    @media (min-width: 768px);
+    @media (min-width: 768px) {
+      .foo {
+      }
+      @media (min-width: 1024px) {
+        .bar {
+        }
+      }
+    }
+    @charset "UTF-8";
+    @layer foo, bar, baz;
+    @custom-media --modern (color), (hover);
+    @namespace 'http://www.w3.org/1999/xhtml';
+    "
+  `)
+
+  expect(toCss(optimizeAst(ast))).toMatchInlineSnapshot(`
+    "@charset "UTF-8";
+    @layer foo, bar, baz;
+    @custom-media --modern (color), (hover);
+    @namespace 'http://www.w3.org/1999/xhtml';
+    "
+  `)
+})

packages/tailwindcss/src/ast.ts

@@ -261,7 +261,9 @@ export function optimizeAst(ast: AstNode[]) {
         for (let child of node.nodes) {
           let nodes: AstNode[] = []
           transform(child, nodes, depth + 1)
-          parent.push(...nodes)
+          if (nodes.length > 0) {
+            parent.push(...nodes)
+          }
         }
       }
 
@@ -271,7 +273,9 @@ export function optimizeAst(ast: AstNode[]) {
         for (let child of node.nodes) {
           transform(child, copy.nodes, depth + 1)
         }
-        parent.push(copy)
+        if (copy.nodes.length > 0) {
+          parent.push(copy)
+        }
       }
     }
 
@@ -297,7 +301,15 @@ export function optimizeAst(ast: AstNode[]) {
       for (let child of node.nodes) {
         transform(child, copy.nodes, depth + 1)
       }
-      parent.push(copy)
+      if (
+        copy.nodes.length > 0 ||
+        copy.name === '@layer' ||
+        copy.name === '@charset' ||
+        copy.name === '@custom-media' ||
+        copy.name === '@namespace'
+      ) {
+        parent.push(copy)
+      }
     }
 
     // AtRoot

packages/tailwindcss/src/compat/plugin-api.test.ts

@@ -3167,8 +3167,6 @@ describe('addUtilities()', () => {
             color: red;
           }
         }
-      }
-      :root, :host {
       }"
     `,
     )

packages/tailwindcss/src/compat/screens-config.test.ts

@@ -655,9 +655,5 @@ test('JS config `screens` can overwrite default CSS `--breakpoint-*`', async ()
   // currently.
   expect(
     compiler.build(['min-sm:flex', 'min-md:flex', 'min-lg:flex', 'min-xl:flex', 'min-2xl:flex']),
-  ).toMatchInlineSnapshot(`
-    ":root, :host {
-    }
-    "
-  `)
+  ).toMatchInlineSnapshot(`""`)
 })