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GhostText
👻 Use your text editor to write in your browser. Everything you type in the editor will be instantly updated in the browser (and vice versa).
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markdown-here
Google Chrome, Firefox, and Thunderbird extension that lets you write email in Markdown and render it before sending.
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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
Another possible solution is to use something like GhostText (https://github.com/fregante/GhostText) in the browser. However, this solution is unworkable because when you open up an email, all the nasty html shows up in the external editor.
MarkDownHere (https://markdown-here.com/) + Email Webapp