Prevent CSS from being duplicated between SSR and prerender

Author: matthewp

packages/astro/src/core/build/plugins/plugin-css.ts

@@ -41,6 +41,8 @@ function rollupPluginAstroBuildCSS(options: PluginOptions): VitePlugin[] {
 	const pagesToCss: Record<string, Record<string, { order: number; depth: number }>> = {};
 	// Map of module Ids (usually something like `/Users/...blog.mdx?astroPropagatedAssets`) to its imported CSS
 	const moduleIdToPropagatedCss: Record<string, Set<string>> = {};
+	// Keep track of CSS that has been bundled to avoid duplication between ssr and prerender.
+	const cssModulesInBundles = new Set();
 
 	const cssBuildPlugin: VitePlugin = {
 		name: 'astro:rollup-plugin-build-css',
@@ -49,6 +51,21 @@ function rollupPluginAstroBuildCSS(options: PluginOptions): VitePlugin[] {
 			return environment.name === 'client' || environment.name === 'ssr' || environment.name === 'prerender';
 		},
 
+		transform(_code, id) {
+			if(isCSSRequest(id)) {
+				// In prerender, don't rebundle CSS that was already bundled in SSR.
+				// Return an empty string here to prevent it.
+				if(this.environment.name === 'prerender') {
+					if(cssModulesInBundles.has(id)) {
+						return {
+							code: ''
+						}
+					}
+				}
+				cssModulesInBundles.add(id);
+			}
+		},
+
 		async generateBundle(_outputOptions, bundle) {
 			// Collect CSS modules that were bundled during SSR build for deduplication in client build
 			if (this.environment?.name === 'ssr' || this.environment?.name === 'prerender') {