diff --git a/e2e/css-composition.spec.ts b/e2e/css-composition.spec.ts
new file mode 100644
index 000000000..54463931b
--- /dev/null
+++ b/e2e/css-composition.spec.ts
@@ -0,0 +1,223 @@
+import { expect, test } from '@playwright/test'
+
+import {
+  codeExtract,
+  exportCanonicalMap,
+  exportClassMap,
+  exportFileMap,
+  exportSheet,
+  getCss,
+  getPrefix,
+  importCanonicalMap,
+  importClassMap,
+  importFileMap,
+  importSheet,
+  isDebug,
+  setDebug,
+  setModuleResolver,
+  setPrefix,
+} from '../bindings/devup-ui-wasm/pkg/index.js'
+
+const api = "import { css } from '@devup-ui/react';"
+const leafRules = `{
+  color: ['orange', 'red', 'purple'], p: 2,
+  _hover: { color: ['tomato', 'red', 'purple'] }
+}`
+const laterRules = `{
+  color: ['blue', null, 'green', null, 'pink'], margin: 3,
+  _hover: { color: ['navy', null, 'lime'] }
+}`
+
+function exportClasses(code: string, name: string): string {
+  const literal = code.match(
+    new RegExp(`export const ${name}\\s*=\\s*("(?:[^"\\\\]|\\\\.)*")\\s*;`),
+  )?.[1]
+  if (literal === undefined) {
+    throw new TypeError(`Expected a compiled literal for ${name}: ${code}`)
+  }
+  const classes: unknown = JSON.parse(literal)
+  if (typeof classes !== 'string') {
+    throw new TypeError(`Expected class string for ${name}`)
+  }
+  expect(classes).toMatch(/^[a-zA-Z0-9_-]+(?: [a-zA-Z0-9_-]+)*$/)
+  return classes
+}
+
+function compile(filename: string, source: string, singleCss: boolean) {
+  const output = codeExtract(
+    filename,
+    source,
+    '@devup-ui/react',
+    'df',
+    singleCss,
+    false,
+    false,
+    {},
+  )
+  try {
+    const css = singleCss ? '' : output.css
+    if (css === undefined) {
+      throw new TypeError(`Expected per-file CSS for ${filename}`)
+    }
+    return { code: output.code, css, dependencies: output.dependencies }
+  } finally {
+    output.free()
+  }
+}
+
+function artifacts(
+  singleCss: boolean,
+  parentSource: string,
+  childSource?: string,
+) {
+  const saved = {
+    sheet: exportSheet(),
+    classes: exportClassMap(),
+    files: exportFileMap(),
+    canonical: exportCanonicalMap(),
+    debug: isDebug(),
+    prefix: getPrefix(),
+  }
+  try {
+    importClassMap({})
+    importFileMap({})
+    importCanonicalMap({})
+    setDebug(false)
+    setPrefix(null)
+    setModuleResolver((specifier: string, importer: string) =>
+      childSource !== undefined &&
+      specifier === './child' &&
+      importer === '/src/parent.ts'
+        ? { path: '/src/child.ts', code: childSource }
+        : undefined,
+    )
+    const child =
+      childSource === undefined
+        ? undefined
+        : compile('/src/child.ts', childSource, singleCss)
+    const parent = compile('/src/parent.ts', parentSource, singleCss)
+    if (child !== undefined) {
+      expect(parent.dependencies).toContain('/src/child.ts')
+    }
+    return {
+      css: singleCss
+        ? getCss(undefined, false)
+        : `${child?.css ?? ''}\n${parent.css}`,
+      parent: exportClasses(parent.code, 'base'),
+      child: exportClasses(
+        child?.code ?? parent.code,
+        child === undefined ? 'leaf' : 'base',
+      ),
+    }
+  } finally {
+    setModuleResolver(null)
+    importSheet(JSON.parse(saved.sheet))
+    importClassMap(JSON.parse(saved.classes))
+    importFileMap(JSON.parse(saved.files))
+    importCanonicalMap(JSON.parse(saved.canonical))
+    setDebug(saved.debug)
+    setPrefix(saved.prefix)
+  }
+}
+
+for (const singleCss of [true, false]) {
+  test.describe(`css composition (${singleCss ? 'single' : 'per-file'} CSS)`, () => {
+    test('later argument wins across the imported W35i leaf', async ({
+      page,
+    }) => {
+      const emitted = artifacts(
+        singleCss,
+        `${api} import { base as child } from './child'; export const base = css(child, { color: 'blue', margin: 3 });`,
+        `${api} export const base = css({ color: 'orange', p: 2 });`,
+      )
+      await page.setContent(`<style>${emitted.css}</style>
+        <div id="parent" class="${emitted.parent}">Parent</div>
+        <div id="child" class="${emitted.child}">Child</div>`)
+      await expect(page.locator('#parent')).toHaveCSS('color', 'rgb(0, 0, 255)')
+      await expect(page.locator('#parent')).toHaveCSS('padding', '8px')
+      await expect(page.locator('#parent')).toHaveCSS('margin', '12px')
+      await expect(page.locator('#child')).toHaveCSS(
+        'color',
+        'rgb(255, 165, 0)',
+      )
+      await expect(page.locator('#child')).toHaveCSS('padding', '8px')
+    })
+
+    for (const chain of [false, true]) {
+      test(`later hover/responsive arguments win across ${chain ? 'a same-module three-level chain' : 'an imported leaf'}`, async ({
+        page,
+      }) => {
+        const emitted = chain
+          ? artifacts(
+              singleCss,
+              `${api}
+              export const leaf = css(${leafRules});
+              const middle = css(leaf, ${laterRules});
+              export const base = css(middle, {
+                color: ['cyan', null, null, null, 'gold'],
+                _hover: { color: ['aqua', null, null, null, 'yellow'] }
+              });`,
+            )
+          : artifacts(
+              singleCss,
+              `${api} import { base as child } from './child'; export const base = css(child, ${laterRules});`,
+              `${api} export const base = css(${leafRules});`,
+            )
+        await page.setContent(`<style>${emitted.css}</style>
+          <div id="parent" class="${emitted.parent}">Parent</div>
+          <div id="child" class="${emitted.child}">Child</div>`)
+        const widths = [
+          [
+            320,
+            chain ? 'rgb(0, 255, 255)' : 'rgb(0, 0, 255)',
+            chain ? 'rgb(0, 255, 255)' : 'rgb(0, 0, 128)',
+            'rgb(255, 165, 0)',
+            'rgb(255, 99, 71)',
+          ],
+          [
+            600,
+            'rgb(255, 0, 0)',
+            'rgb(255, 0, 0)',
+            'rgb(255, 0, 0)',
+            'rgb(255, 0, 0)',
+          ],
+          [
+            900,
+            'rgb(0, 128, 0)',
+            'rgb(0, 255, 0)',
+            'rgb(128, 0, 128)',
+            'rgb(128, 0, 128)',
+          ],
+          [
+            1400,
+            chain ? 'rgb(255, 215, 0)' : 'rgb(255, 192, 203)',
+            chain ? 'rgb(255, 255, 0)' : 'rgb(0, 255, 0)',
+            'rgb(128, 0, 128)',
+            'rgb(128, 0, 128)',
+          ],
+        ] as const
+        for (const [
+          width,
+          parentColor,
+          hoverColor,
+          childColor,
+          childHoverColor,
+        ] of widths) {
+          await page.setViewportSize({ width, height: 600 })
+          await page.mouse.move(width - 1, 599)
+          await expect(page.locator('#parent')).toHaveCSS('color', parentColor)
+          await expect(page.locator('#parent')).toHaveCSS('padding', '8px')
+          await expect(page.locator('#parent')).toHaveCSS('margin', '12px')
+          await expect(page.locator('#child')).toHaveCSS('color', childColor)
+          await page.locator('#parent').hover()
+          await expect(page.locator('#parent')).toHaveCSS('color', hoverColor)
+          await page.locator('#child').hover()
+          await expect(page.locator('#child')).toHaveCSS(
+            'color',
+            childHoverColor,
+          )
+        }
+      })
+    }
+  })
+}
diff --git a/libs/extractor/src/lib.rs b/libs/extractor/src/lib.rs
index a1b30ced1..ce7e3e3aa 100644
--- a/libs/extractor/src/lib.rs
+++ b/libs/extractor/src/lib.rs
@@ -15299,6 +15299,130 @@ export const d = css(ordered, danger, called);",
         }
     }
 
+    fn assert_composed_export_atoms(code: &str, export: &str, expected: &[&str]) {
+        let declaration = format!("export const {export} = \"");
+        let classes = code
+            .split_once(&declaration)
+            .expect("compiled export must be a static class string")
+            .1
+            .split_once('"')
+            .unwrap()
+            .0;
+        let mut actual: Vec<_> = classes
+            .split_whitespace()
+            .map(|class| class.trim_end_matches("-a"))
+            .collect();
+        actual.sort_unstable();
+        let mut expected = expected.to_vec();
+        expected.sort_unstable();
+        assert_eq!(actual, expected, "{code}");
+    }
+
+    #[rstest]
+    #[case(
+        "{ color: 'orange', p: 2 }",
+        "{ color: 'blue', margin: 3 }",
+        &["color-0-blue--255", "padding-0-8px--255", "margin-0-12px--255"]
+    )]
+    #[case(
+        "{ color: ['orange', 'red', 'purple'], p: 2, _hover: { color: ['orange', 'red', 'purple'] } }",
+        "{ color: ['blue', null, 'green', null, 'pink'], margin: 3, _hover: { color: ['blue', null, 'green'] } }",
+        &[
+            "color-0-blue--255", "color-1-red--255", "color-2-green--255",
+            "color-4-pink--255", "padding-0-8px--255", "margin-0-12px--255",
+            "color-0-blue-_a__c_hover-255", "color-1-red-_a__c_hover-255",
+            "color-2-green-_a__c_hover-255"
+        ]
+    )]
+    #[serial]
+    fn test_css_later_argument_wins_across_leaf(
+        #[case] earlier: &'static str,
+        #[case] later: &str,
+        #[case] expected: &[&str],
+        #[values(false, true)] imported: bool,
+        #[values(false, true)] single_css: bool,
+    ) {
+        reset_class_map();
+        reset_file_map();
+        let modules: &[(&str, &str)] = &[
+            (
+                "/src/child.ts",
+                "import { css } from '@devup-ui/react'; export const base = css({ color: 'orange', p: 2 });",
+            ),
+            (
+                "/src/responsive.ts",
+                "import { css } from '@devup-ui/react'; export const base = css({ color: ['orange', 'red', 'purple'], p: 2, _hover: { color: ['orange', 'red', 'purple'] } });",
+            ),
+        ];
+        let leaf = if earlier.contains("_hover") {
+            "responsive"
+        } else {
+            "child"
+        };
+        let binding = if imported {
+            format!("import {{ base as child }} from './{leaf}';")
+        } else {
+            format!("const child = css({earlier});")
+        };
+        let source = format!(
+            "import {{ css }} from '@devup-ui/react'; {binding} export const base = css(child, {later});"
+        );
+        let resolver = memory_resolver(modules);
+        css::debug::set_debug(true);
+        let output = extract_with_modules(
+            "/src/parent.ts",
+            &source,
+            ExtractOption {
+                single_css,
+                ..ExtractOption::default()
+            },
+            false,
+            &resolver,
+        );
+        css::debug::set_debug(false);
+        let output = output.unwrap();
+        assert_composed_export_atoms(&output.code, "base", expected);
+        if imported {
+            assert!(output.dependencies.contains(&format!("/src/{leaf}.ts")));
+        }
+    }
+
+    #[rstest]
+    #[serial]
+    fn test_css_later_argument_wins_across_three_levels(#[values(false, true)] single_css: bool) {
+        reset_class_map();
+        reset_file_map();
+        let resolver = memory_resolver(&[]);
+        css::debug::set_debug(true);
+        let output = extract_with_modules(
+            "/src/chain.ts",
+            "import { css } from '@devup-ui/react';
+const leaf = css({ color: ['orange', 'red', 'purple'], p: 2, _hover: { color: ['orange', 'red', 'purple'] }, styleOrder: 3 });
+const middle = css(leaf, { color: ['blue', null, 'green'], margin: 3, _hover: { color: ['blue', null, 'green'] } });
+export const base = css(middle, { color: ['cyan', null, null, null, 'pink'], _hover: { color: 'cyan' } });",
+            ExtractOption { single_css, ..ExtractOption::default() },
+            false,
+            &resolver,
+        );
+        css::debug::set_debug(false);
+        let output = output.unwrap();
+        assert_composed_export_atoms(
+            &output.code,
+            "base",
+            &[
+                "color-0-cyan--255",
+                "color-1-red--3",
+                "color-2-green--255",
+                "color-4-pink--255",
+                "padding-0-8px--3",
+                "margin-0-12px--255",
+                "color-0-cyan-_a__c_hover-255",
+                "color-1-red-_a__c_hover-3",
+                "color-2-green-_a__c_hover-255",
+            ],
+        );
+    }
+
     // Each Tailwind class becomes the classes of its styles; every other class,
     // and every class that runs into an interpolation, stays as written
     #[rstest]