Fylgja is tested and supported in all modern browsers (Chrome, Firefox, Edge, and Safari).
The main goal is that you can use Fylgja, without having to use any extra tooling to make it work in the browser. So no need to have tools like postcss-preset-env or autoprefixer by default.
We do recommend tools like these, to make it easier to use newer CSS features.
More on it in our other guide for PostCSS tools
That said there are cases that some of the Fylgja's CSS Components may also include newer CSS/HTML features, That may need tooling or an polyfill to work.
Fylgja strives to support at least a minimal of 2 major versions back, this does not include older browsers like IE11.
But in case one of Fylgja's components does not match your requirement for browser support, you can avoid it, since each Fylgja CSS Component can be used separately, meaning you decide what you need from Fylgja.