diff --git a/packages/plugin-legacy/src/index.ts b/packages/plugin-legacy/src/index.ts
index ea96abaa1330ca..898f7ab4cf7558 100644
--- a/packages/plugin-legacy/src/index.ts
+++ b/packages/plugin-legacy/src/index.ts
@@ -327,6 +327,17 @@ function viteLegacyPlugin(options: Options = {}): Plugin[] {
       }
     },
 
+    renderStart(opts) {
+      // @ts-ignore
+      // In the `renderChunk` hook,
+      // we skip inlining CSS for the entry chunk because a separate CSS file
+      // would've been already linked in index.html (see #9761).
+      // But if `genModern` is set to false, the CSS still should be inlined.
+      // If in the future vite doesn't inline CSS any more (as #9920 suggests),
+      // this flag can be safely deleted.
+      opts.__vite_inline_entry_css__ = !genModern
+    },
+
     async renderChunk(raw, chunk, opts) {
       if (config.build.ssr) {
         return null
diff --git a/packages/vite/src/node/plugins/css.ts b/packages/vite/src/node/plugins/css.ts
index aebb3286584662..eb02d3085393ff 100644
--- a/packages/vite/src/node/plugins/css.ts
+++ b/packages/vite/src/node/plugins/css.ts
@@ -563,7 +563,8 @@ export function cssPostPlugin(config: ResolvedConfig): Plugin {
           // the legacy build should avoid inserting entry CSS modules here, they
           // will be collected into `chunk.viteMetadata.importedCss` and injected
           // later by the `'vite:build-html'` plugin into the `index.html`
-          if (chunk.isEntry) {
+          // @ts-ignore
+          if (chunk.isEntry && !config.build.lib && !opts.__vite_inline_entry_css__) {
             return null
           }
           chunkCSS = await finalizeCss(chunkCSS, true, config)