diff --git a/src/download.js b/src/download.js
index 3b15a68..d7e0a9f 100644
--- a/src/download.js
+++ b/src/download.js
@@ -31,16 +31,7 @@ async function createZipWithSingleFile ({ htmlContent, cssContent, jsContent })
 async function createZipWithMultipleFiles ({ htmlContent, cssContent, jsContent }) {
   const zip = await getZip()
 
-  const indexHtml = `<!DOCTYPE html>
-<html lang="en">
-  <head>
-    <link type="text/css" rel="stylesheet" href="style.css"/>
-  </head>
-  <body>
-    ${htmlContent}
-    <script type="module" src="script.js"></script>
-  </body>
-</html>`
+  const indexHtml = buildIndexHtml(htmlContent)
 
   return await zip([
     { name: 'style.css', input: cssContent },
@@ -49,6 +40,77 @@ async function createZipWithMultipleFiles ({ htmlContent, cssContent, jsContent
   ]).blob()
 }
 
+function buildIndexHtml (html) {
+  const hasDoctype = /<!doctype\s+html[^>]*>/i.test(html)
+  const hasHtml = /<html[\s>]/i.test(html)
+  const hasHead = /<head[\s>]/i.test(html)
+  const hasBody = /<body[\s>]/i.test(html)
+  const hasStyleLink = /<link[^>]*href=["']style\.css["'][^>]*>/i.test(html)
+  const hasScript = /<script[^>]*src=["']script\.js["'][^>]*>/i.test(html)
+
+  let result = html
+
+  // Add script before </body> or at the end
+  if (!hasScript) {
+    if (hasBody) {
+      result = result.replace(/<\/body>/i, '    <script type="module" src="script.js"></script>\n  </body>')
+    } else {
+      result = result + '\n    <script type="module" src="script.js"></script>'
+    }
+  }
+
+  // Add stylesheet link inside <head> or store for later
+  if (!hasStyleLink) {
+    if (hasHead) {
+      result = result.replace(/<head([^>]*)>/i, '<head$1>\n    <link type="text/css" rel="stylesheet" href="style.css"/>')
+    }
+    // If no <head>, it will be added below with the link included
+  }
+
+  // Add <head> if missing
+  if (!hasHead) {
+    const headBlock = '  <head>\n    <link type="text/css" rel="stylesheet" href="style.css"/>\n  </head>'
+    if (hasBody) {
+      result = result.replace(/<body/i, headBlock + '\n  <body')
+    } else if (hasHtml) {
+      result = result.replace(/<html([^>]*)>/i, '<html$1>\n' + headBlock)
+    } else {
+      result = headBlock + '\n' + result
+    }
+  }
+
+  // Wrap in <body> if missing
+  if (!hasBody) {
+    if (hasHtml) {
+      // Find content after </head> and before </html>, wrap it in <body>
+      result = result.replace(/(<\/head>)([\s\S]*?)(<\/html>)/i, '$1\n  <body>\n    $2\n  </body>\n$3')
+    } else {
+      // Find content after </head> and wrap the rest in <body>
+      const headEnd = result.indexOf('</head>')
+      if (headEnd !== -1) {
+        const afterHead = headEnd + '</head>'.length
+        const before = result.slice(0, afterHead)
+        const after = result.slice(afterHead)
+        result = before + '\n  <body>' + after + '\n  </body>'
+      } else {
+        result = '  <body>\n' + result + '\n  </body>'
+      }
+    }
+  }
+
+  // Wrap in <html> if missing
+  if (!hasHtml) {
+    result = '<html lang="en">\n' + result + '\n</html>'
+  }
+
+  // Add DOCTYPE if missing
+  if (!hasDoctype) {
+    result = '<!DOCTYPE html>\n' + result
+  }
+
+  return result
+}
+
 function generateZip ({ zipBlob, zipFileName }) {
   console.log({ zipBlob, zipFileName })
   const element = window.document.createElement('a')
