If you've tested on an iPhone, you may notice that the website doesn't respect the font size you've chosen in settings. This is because iPhones don't make the default font larger or smaller, but rather switch out the font itself (or something like that, I don't fully understand it). To solve this problem, you need to include:
@supports (font: -apple-system-body) {
    html {
        font: -apple-system-body;
    }
}
Read more