Hi
I have create a template from beefree.io but can't upload to mailwizz. Event if I copy only html, after save, html page is blank.
If you use a standards' compliant html editor, it will, on saving a file, fix many, if not all, such errors.
Some offer features like:
'remove comments' (incl all html comments re dyn web, layout, scripts, other)
'remove whitespace' (leading, all)
'remove unused content' (merge adj, empty, unused tags)
etc
It is always a good idea to clean, adapt, and test those html email templates, as you are doing