Why Sapper omits clossing HTML tags?

When I clone the official Sapper template, install dependencies and export it

~$ degit "sveltejs/sapper-template#webpack" sapper-template
~$ cd sapper-template/
~/sapper-template$ npm install
~/sapper-template$ npm run export

and then inspect the generated index.html, the closing </body> and </html> tags are absent.

...
<body>
    ...
    <script src=/client/4def89e12e93fe68fd0d/main.js> </script>
// EOF

Why does Sapper generate formally invalid HTML?