Mosaico Editor is the first creator of this kind of email email template (AFAIK).
You can find a free deployment (also working as a demo version) at http://mosaico.io , and you can get sources at https://github.com/voidlabs/mosaico
I select blocks from a set defined by a pattern >, then you fill in the content and change its styles in the style of WYSIWYG . If you are in a large window, you can also preview the preview for the mobile version.
The master template determines what the blocks are, what you can edit and what you can style, and contains any html trick to make it compatible with most clients: this means that you can greatly change the editorβs behavior by simply writing a new master template.
This is 99% javascript (IE10 + and any other modern browser) and depends on server functions only for "final insertion" and "upload / change image"
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