Improve CSS import ordering, fix empty CSS outputs (#2081)

Fixes #2060

4 files changed, 50 insertions, 12 deletions

diff --git a/.changeset/clever-suits-hide.md b/.changeset/clever-suits-hide.md
new file mode 100644
index 000000000..31fdb518b
--- /dev/null
+++ b/.changeset/clever-suits-hide.md
@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Bugfix: CSS import ordering, empty CSS output on build
diff --git a/packages/astro/src/vite-plugin-build-css/index.ts b/packages/astro/src/vite-plugin-build-css/index.ts
index d46f56d13..2989faee2 100644
--- a/packages/astro/src/vite-plugin-build-css/index.ts
+++ b/packages/astro/src/vite-plugin-build-css/index.ts
@@ -124,6 +124,8 @@ export function rollupPluginAstroBuildCSS(options: PluginOptions): VitePlugin {
         }
       }
 
+      if (!chunkCSS) return null; // don’t output empty .css files
+
       if (isPureCSS) {
         const { code: minifiedCSS } = await esbuild.transform(chunkCSS, {
           loader: 'css',
diff --git a/packages/astro/src/vite-plugin-build-html/index.ts b/packages/astro/src/vite-plugin-build-html/index.ts
index 7650c0224..cdc5c1877 100644
--- a/packages/astro/src/vite-plugin-build-html/index.ts
+++ b/packages/astro/src/vite-plugin-build-html/index.ts
@@ -55,6 +55,7 @@ export function rollupPluginAstroBuildHTML(options: PluginOptions): VitePlugin {
   const astroAssetMap = new Map<string, Promise<Buffer>>();
 
   const cssChunkMap = new Map<string, string[]>();
+  const pageStyleImportOrder: string[] = [];
 
   return {
     name: PLUGIN_NAME,
@@ -176,6 +177,11 @@ export function rollupPluginAstroBuildHTML(options: PluginOptions): VitePlugin {
             const jsSource = assetImports.map((sid) => `import '${sid}';`).join('\n');
             astroPageStyleMap.set(pageStyleId, jsSource);
             assetInput.add(pageStyleId);
+
+            // preserve asset order in the order we encounter them
+            for (const assetHref of assetImports) {
+              if (!pageStyleImportOrder.includes(assetHref)) pageStyleImportOrder.push(assetHref);
+            }
           }
         }
       }
@@ -260,17 +266,43 @@ export function rollupPluginAstroBuildHTML(options: PluginOptions): VitePlugin {
         assetIdMap.set(assetPath, referenceId);
       }
 
+      // Sort CSS in order of appearance in HTML (pageStyleImportOrder)
+      // This is the “global ordering” used below
+      const sortedCSSChunks = [...pureCSSChunks];
+      sortedCSSChunks.sort((a, b) => {
+        let aIndex = Math.min(
+          ...Object.keys(a.modules).map((id) => {
+            const i = pageStyleImportOrder.findIndex((url) => id.endsWith(url));
+            return i >= 0 ? i : Infinity; // if -1 is encountered (unknown order), move to the end (Infinity)
+          })
+        );
+        let bIndex = Math.min(
+          ...Object.keys(b.modules).map((id) => {
+            const i = pageStyleImportOrder.findIndex((url) => id.endsWith(url));
+            return i >= 0 ? i : Infinity;
+          })
+        );
+        return aIndex - bIndex;
+      });
+      const sortedChunkNames = sortedCSSChunks.map(({ fileName }) => fileName);
+
       // Create a mapping of chunks to dependent chunks, used to add the proper
       // link tags for CSS.
-      for (const chunk of pureCSSChunks) {
-        const chunkReferenceIds: string[] = [];
-        for (const [specifier, chunkRefID] of chunkToReferenceIdMap.entries()) {
-          if (chunk.imports.includes(specifier) || specifier === chunk.fileName) {
-            chunkReferenceIds.push(chunkRefID);
-          }
+      for (const chunk of sortedCSSChunks) {
+        const chunkModules = [chunk.fileName, ...chunk.imports];
+        // For each HTML output, sort CSS in HTML order Note: here we actually
+        // want -1 to be first. Since the last CSS “wins”, we want to load
+        // “unknown” (-1) CSS ordering first, followed by “known” ordering at
+        // the end so it takes priority
+        chunkModules.sort((a, b) => sortedChunkNames.indexOf(a) - sortedChunkNames.indexOf(b));
+
+        const referenceIDs: string[] = [];
+        for (const chunkID of chunkModules) {
+          const referenceID = chunkToReferenceIdMap.get(chunkID);
+          if (referenceID) referenceIDs.push(referenceID);
         }
-        for (const [id] of Object.entries(chunk.modules)) {
-          cssChunkMap.set(id, chunkReferenceIds);
+        for (const id of Object.keys(chunk.modules)) {
+          cssChunkMap.set(id, referenceIDs);
         }
       }
 
diff --git a/packages/astro/test/astro-css-bundling-import.test.js b/packages/astro/test/astro-css-bundling-import.test.js
index ea605143e..ae9b085d6 100644
--- a/packages/astro/test/astro-css-bundling-import.test.js
+++ b/packages/astro/test/astro-css-bundling-import.test.js
@@ -10,7 +10,7 @@ describe('CSS Bundling (ESM import)', () => {
     await fixture.build();
   });
 
-  it.skip('CSS output in import order', async () => {
+  it('CSS output in import order', async () => {
     // note: this test is a little confusing, but the main idea is that
     // page-2.astro contains all of page-1.astro, plus some unique styles.
     // we only test page-2 to ensure the proper order is observed.
@@ -29,11 +29,10 @@ describe('CSS Bundling (ESM import)', () => {
     expect(css.indexOf('p{color:green}')).to.be.greaterThan(css.indexOf('p{color:red}'));
 
     // test 2: insure green comes after blue (page-1.css)
-    expect(css.indexOf('p{color:green}')).to.be.greaterThan(css.indexOf('p{color:red}'));
+    expect(css.indexOf('p{color:green}')).to.be.greaterThan(css.indexOf('p{color:#00f}'));
   });
 
-  // TODO: need more investigation to fix this
-  it.skip('no empty CSS files', async () => {
+  it('no empty CSS files', async () => {
     for (const page of ['/page-1/index.html', '/page-2/index.html']) {
       const html = await fixture.readFile(page);
       const $ = cheerio.load(html);