diff --git a/scripts/build-dist.mjs b/scripts/build-dist.mjs
index 0153dd4a..74e19bda 100644
--- a/scripts/build-dist.mjs
+++ b/scripts/build-dist.mjs
@@ -38,7 +38,11 @@ function copyFile(src, dest) {
   if (fs.existsSync(src)) {
     fs.copyFileSync(src, dest);
   } else {
-    console.warn(`Warning: Source file ${src} does not exist.`);
+    const message = `Error: Source file ${src} does not exist.`;
+    console.error(message);
+    if (['_headers', '_redirects', 'index.html'].includes(path.basename(src))) {
+      throw new Error(message);
+    }
   }
 }
 
@@ -74,6 +78,19 @@ function main() {
     copyFile(file, path.join(DIST, file));
   }
 
+  // Verify critical CSS generation
+  const generatedCss = path.join(DIST, 'css', 'tailwind.generated.css');
+  if (!fs.existsSync(generatedCss) && !fs.existsSync(path.join('css', 'tailwind.generated.css'))) {
+     // tailwind might output to dist/css OR css/ then we copy.
+     // The command is: -o css/tailwind.generated.css
+     // So it should be in source 'css/', then copied by copyDir('css'...)
+     // Let's verify it exists in source 'css/'
+     if (!fs.existsSync(path.join('css', 'tailwind.generated.css'))) {
+         console.error('Error: css/tailwind.generated.css was not generated.');
+         process.exit(1);
+     }
+  }
+
   console.log('Copying directories...');
   for (const dir of DIRS_TO_COPY) {
     copyDir(dir, path.join(DIST, dir));