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.

There is this tool if you use stylelint for linting: https://github.com/RJWadley/stylelint-no-unsupported-browser-features

Given that your website is very simple, I assume it would be a pretty big rabbit-hole to dig into.

It wouldn't reveal the oldest version of each browser compatible with your website, but it would check your css against a predefined list, and alert you if you use something that is not compatible.

Otherwise, if you are ok with checking manually, check out BrowserStack: https://www.browserstack.com/ It's neither free or open source, but it simplifies testing on many browsers. There is a free plan as well.

Browserling can help maybe (i'm not affiliated lol) https://www.browserling.com/

Hmm, I have a blink and a gecko browser on desktop and mobile, and open the site on both to check how it's being displayed :S

I hear a lot of people complaining about sites breaking on Firefox, but I never experienced that, only on secure forks that removed canvas, webgl and webgpu.

I have a few static sites as well and I use JS. The only thing I noticed changing, that can push elements weirdly, is scroll bars, buttons, the default audio player... but you can edit those with css to look similar on both browsers.