diff --git a/packages/trace-viewer/src/snapshotServer.ts b/packages/trace-viewer/src/snapshotServer.ts
index c3c2f5a6247d7..b1dd371cb373f 100644
--- a/packages/trace-viewer/src/snapshotServer.ts
+++ b/packages/trace-viewer/src/snapshotServer.ts
@@ -85,7 +85,9 @@ export class SnapshotServer {
       contentType = `${contentType}; charset=utf-8`;
 
     const headers = new Headers();
-    headers.set('Content-Type', contentType);
+    // "x-unknown" in the har means "no content type".
+    if (contentType !== 'x-unknown')
+      headers.set('Content-Type', contentType);
     for (const { name, value } of resource.response.headers)
       headers.set(name, value);
     headers.delete('Content-Encoding');
diff --git a/packages/trace-viewer/src/traceModel.ts b/packages/trace-viewer/src/traceModel.ts
index 0fc1a73efa76c..1248dde967a07 100644
--- a/packages/trace-viewer/src/traceModel.ts
+++ b/packages/trace-viewer/src/traceModel.ts
@@ -114,9 +114,11 @@ export class TraceModel {
 
   async resourceForSha1(sha1: string): Promise<Blob | undefined> {
     const blob = await this._backend.readBlob('resources/' + sha1);
-    if (!blob)
-      return;
-    return new Blob([blob], { type: this._resourceToContentType.get(sha1) || 'application/octet-stream' });
+    const contentType = this._resourceToContentType.get(sha1);
+    // "x-unknown" in the har means "no content type".
+    if (!blob || contentType === undefined || contentType === 'x-unknown')
+      return blob;
+    return new Blob([blob], { type: contentType });
   }
 
   storage(): SnapshotStorage {
diff --git a/tests/library/trace-viewer.spec.ts b/tests/library/trace-viewer.spec.ts
index e8471c16d6c78..3af586d67fe5e 100644
--- a/tests/library/trace-viewer.spec.ts
+++ b/tests/library/trace-viewer.spec.ts
@@ -1439,3 +1439,15 @@ test('should show baseURL in metadata pane', {
   await traceViewer.showMetadataTab();
   await expect(traceViewer.metadataTab).toContainText('baseURL:https://example.com');
 });
+
+test('should serve css without content-type', async ({ page, runAndTrace, server }) => {
+  server.setRoute('/one-style.css', (req, res) => {
+    res.writeHead(200);
+    res.end(`body { background: red }`);
+  });
+  const traceViewer = await runAndTrace(async () => {
+    await page.goto(server.PREFIX + '/one-style.html');
+  });
+  const snapshotFrame = await traceViewer.snapshotFrame('page.goto');
+  await expect(snapshotFrame.locator('body')).toHaveCSS('background-color', 'rgb(255, 0, 0)', { timeout: 0 });
+});