Can I use HTML

[#1]

Yes, we do support HTML:

1. Copy the source code from your emails in MailChimp (or whatever tool you use to create emails in HTML)
2. Hit the source button in your template editor
3. Paste the source code there
4. Hit save

If you are seeing the error "Error - to remove the style/java/html tags" this means you have some styling that we do not support.  You should search the Source code for the word style and delete everything in the source code from <style> through </style>