Ask HN: How to build the simplest of websites that works on all mobile sizes?

I want to build a very simple web app with only one or two simple forms, but I dread the task of trying to make it look readable on the vast array of mobile devices and screen resolutions.

Is there any solution to this? Is there any front end framework that specifically makes it easy to build something that looks OK on most mobile devices/resolutions.

I use bootstrap 4 but that's not really a complete answer is it?