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.
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.