-
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.
What an excellent resource! (And yes Outlook is a pain!)
We've tried building email templates for notifications for our apps where I work, and it has typically been a pain. We have since swapped to using mjml (https://mjml.io/) to build the templates, and it's working wonders. The output seems the be the most compatible with all different devices that we've tested on.
The other tool we enjoy using is Litmus (litmus.com), which allows you to throw in an email template and see what it looks like on all kinds of apps and devices.
Both of these have been really invaluable to designing emails for our apps.
https://caniuse.com/ is a popular tool to check what web features are working across different browsers - "can you use this and assume that it will work for others".