This is an old draft from 2020. It’s not finished, but I am publishing it anyway.
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Initially I was not aware how limiting AMP is. I just assumed that you can make a website AMP compliant by only minor modifications. So I started out by looking for a website authoring tool with Responsive Design.
“19 Best Free Responsive Website Builder Software 2018 – Colorlib”
it is an off-line application, unlike most of the apps of the same kind that are web apps. Mobirise does have an experimental web app version at 8b.com (announcement), but clearly this is not ready for prime time. The fact you cannot export authoring data is a clear deal breaker for me.
AMP Website Builder as if it was a separate app, but it isn’t.
Supposedly based on Bootstrap (front-end framework) – Wikipedia and there are tons of free Bootstrap themes available on the Web (some are AMP-compliant), but there is no way for you to utilize them by yourself.
DeltaPi softwares provides Mobirise extensions, including a code editor, and Mobitise utilities. DeltaPi Home page
The original code editor by Mobirise costs as much as $69, and I believe it is a yearly recurring fee.
AMP Test – Google Search Console
“Japanese Web Fonts Available for Use on AMP-Compliant Websites“