h8every1

Swiffy output (i.e. HML5 banner) itself cannot be saved using Code Mode in Chrome browser.
It generates a error:

The XSS Auditor refused to execute a script in <your-url> because its source code was found within the request. The auditor was enabled as the server sent neither an 'X-XSS-Protection' nor 'Content-Security-Policy' header.'

You can send those headers on your server but I personally think, that this is a bad idea.

The other solution I found for now is saving HTML5 banners as separate html-files and include them using iframe in Code Mode.

And what if you want to use the same template on several categories?
Say, 4 categories of 20 on the site? WP doesn’t support this.