I’m not sure if this will help but I have written web pages for a web site that were responsive because I simply loaded different .html files depending on the size of the devices screen. I do not recommend this procedure for making a responsive web site.
But this method is not a “Web App” that uses a manifest to allow the “Web Page” to run when no Internet is available. For example, news.google.com is a responsive web site. Try it, run it in your PC browser and resize the browser. It’s responsive. But disconnect from the internet and its a no show. This feature is extremely important to my applications written in App Studio.