I have some experience in this, but none of it should be viewed as infallible.
As Steve mentioned above there are multiple online email creators, which may work fine for you.
A few things to keep in mind. Does your email need to be responsive? The problem is not all email clients can “read” responsive emails. View this chart which shows which clients support responsive design.
Another issue is you mention creating templates. Is this a template for yourself or a designer? If so you may have more flexibility. But if it’s for an “average” end user who doesn’t know HTML you may be out of luck.
This chart helps you to understand what is and isn’t generally possible in email.
Bottom line, if you’re coding it, keep it generally simple, unless you know that the majority of your viewers are viewing on a certain platform that supports more functionality.
I work for a company that uses Outlook for their email client, which generally does not do real well with anything but simple HTML. At the end of the day, we create templates for average non-technical users to use. We defaulted to setting our templates in MS Word. Not pretty, but functional. The main benefit was it is WYSIWYG. We created the email as a table in Word. Outlook seems to do well with MS Word tables. So, no web fonts, only system fonts unless it’s a corporate environment where you know your entire org has corporate fonts on their system. Also the graphics need to be sized at either 100% or in our case we use 50% size to ensure they are clear and not fuzzy. You can check that by right clicking on images and choosing size and position and verify scaling is at 100% or 50%.
And lastly, any time you give a template to someone in something like MS Word, be aware that it can and most likely will break, because if the end user isn’t educated and they start messing with it, inserting graphics they find “somewhere online” and inserting word art and odd fonts, well, you’ll be SOL.
Oh, and always test your template before providing the final to anyone.