diff --git a/packages/client/modules/src/client/system.ts b/packages/client/modules/src/client/system.ts
index 29da7c4163..6597f54afc 100644
--- a/packages/client/modules/src/client/system.ts
+++ b/packages/client/modules/src/client/system.ts
@@ -63,15 +63,20 @@ function chunkUrl(row: BootModuleRow, fileName: string, rev: string): string {
   return `${url.slice(0, resourceStart)}/${row.id}/${fileName}?${url.slice(revisionStart + 1)}`
 }
 
+/** Untagged <style> elements present before a factory runs; the factory must not claim them. */
+const untaggedStyles = (): ReadonlySet<Element> =>
+  typeof document === 'undefined' ? new Set() : new Set(document.querySelectorAll('style:not([data-plugin])'))
+
 /**
  * Claim and inventory the <style> tags a factory injected during
  * materialization: preset-emitted tags arrive pre-tagged with data-plugin;
- * any untagged tag is claimed for the materializing plugin (HMR bookkeeping).
+ * an untagged tag that was absent from `before` is claimed for the
+ * materializing plugin (HMR bookkeeping).
  */
-const claimStyles = (id: string): string[] => {
+const claimStyles = (id: string, before: ReadonlySet<Element>): string[] => {
   if (typeof document === 'undefined') return []
-  for (const el of document.querySelectorAll('style:not([data-plugin])')) {
-    el.setAttribute('data-plugin', id)
+  for (const el of untaggedStyles()) {
+    if (!before.has(el)) el.setAttribute('data-plugin', id)
   }
   const owned: string[] = []
   for (const el of document.querySelectorAll(`style[data-plugin=${JSON.stringify(id)}]`)) {
@@ -300,13 +305,15 @@ export class ClientModuleSystem implements ClientModuleLoader {
       throw new Error(`client-modules: require cycle through "${id}" (factory-form CJS cannot deliver partial exports)`)
     }
     this.materializing.add(id)
+    const before = untaggedStyles()
     try {
       const edges = new Set<string>()
       const exports = registered.factory(this.makeRequire(ownerId, edges))
-      const record: ClientModuleRecord = { id, exports, styles: claimStyles(ownerId), edges }
+      const record: ClientModuleRecord = { id, exports, styles: claimStyles(ownerId, before), edges }
       this.loadCache.set(id, record)
       return record
     } catch (error) {
+      claimStyles(ownerId, before)
       removeOwnedStyles(ownerId)
       throw error
     } finally {
diff --git a/packages/client/modules/tests/loader.client.spec.ts b/packages/client/modules/tests/loader.client.spec.ts
index 5f06f62aa6..a5285e1c54 100644
--- a/packages/client/modules/tests/loader.client.spec.ts
+++ b/packages/client/modules/tests/loader.client.spec.ts
@@ -832,6 +832,37 @@ describe('style claiming', () => {
     expect(foreign.getAttribute('data-plugin')).toBe('other')
   })
 
+  it('leaves untagged style tags inserted before materialization unclaimed', async () => {
+    const earlier = document.createElement('style')
+    document.head.appendChild(earlier)
+    const b = bench([row('a')], { a: () => ({}) })
+    await b.loader.import('a', '', {})
+    removeOwnedStyles('a')
+    expect(earlier.isConnected).toBe(true)
+    expect(earlier.hasAttribute('data-plugin')).toBe(false)
+  })
+
+  it('assigns an untagged style to the factory that inserted it before a nested require', async () => {
+    const outer = document.createElement('style')
+    const b = bench([row('b'), row('a', { external: ['b'] })], {
+      a: (require) => {
+        document.head.appendChild(outer)
+        require('b')
+        return {}
+      },
+      b: () => ({}),
+    })
+    await b.loader.import('a', '', {})
+    expect(outer.getAttribute('data-plugin')).toBe('a')
+  })
+
+  it('removes untagged styles inserted by a factory that throws', async () => {
+    const leaked = document.createElement('style')
+    const b = bench([row('a')], { a: () => { document.head.appendChild(leaked); throw new Error('factory exploded') } })
+    await expect(b.loader.import('a', '', {})).rejects.toThrow('factory exploded')
+    expect(leaked.isConnected).toBe(false)
+  })
+
   it('materialization without a document skips the style inventory', async () => {
     const b = bench([row('a')], { a: () => ({}) })
     vi.stubGlobal('document', undefined)