A polyfill for font-family: system-ui; in CSS

system-ui-polyfill.js

(() => {
    let div = document.createElement("div");
    div.style.font = "menu"
    document.body.appendChild(div)
    let font = (getComputedStyle(div).fontFamily)
    div.remove()
    div = null;
    let sele = document.createElement("style");
    sele.textContent = `@font-face {
  font-family: 'system-ui';
  src: local('${font}');
}`;
    document.body.appendChild(sele)
})();