dynamic_font_loading.js

function loadFontStyles() {
  if (!window.localStorage) { return; }

  var cssText;
  if (cssText = localStorage.getItem('font-style-cache')) {
    injectFontCSS(cssText);
  } else {
    fetch('/css/fonts.css').then(function(cssText) {
      localStorage.setItem('font-style-cache', cssText);
      return cssText;
    }).then(injectFontCSS(cssText));
  }
}

function injectFontCSS(cssText) {
  var style = document.createElement('style');
  style.type = 'text/css';
  style.styleSheet.cssText = cssText;
  document.head.appendChild(style);
}