

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.






~~ No, Mozilla uses Mullvad as a partner, they’re a serious and nolog VPN provider.
https://mullvad.net/en/blog/mullvad-vpn-was-subject-to-a-search-warrant-customer-data-not-compromised ~~ EDIT: Didn’t read the article, the free VPN won’t be using Mullvad apparently.