I have made a website in simple HTML and CSS (no JS).
I hope it would be compatible with as many browsers as possible.
Is there a free and open source solution that can test and reveal the oldest version of each browser compatible with my site? I want to be able to say something like: This site is compatible with FF v60, Chromium v50, Safari vxx, etc.
Or make a compatibility table so users can know that this was tested.


Most browser compatibility issues come from JavaScript, not HTML/CSS. Unless you want to support ancient/dead browsers (like Internet Explorer or non-HTML5 browsers), it’s very likely there won’t be any issues.
MDN has a compatibility table for every HTML tag and CSS property. For a simple static site I would just manually check there.