загрузка шрифтов через FontFaceObserver.

fontfaceset.js

let fontFace = new FontFace("MyWebFont", "url('MyWebFont.woff2') format('woff2'), url('MyWebFont.woff') format('woff')");

fontFace.load().then(function(loadedFontFace) {
    document.fonts.add(loadedFontFace);
    document.getElementById("target").style.fontFamily = "MyWebFont";
});

fontloader.js

;(function(window, document, undefined) {
    'use strict';

    var fonts = [
        {
            name: 'Open Sans',
            weight: 'normal',
            style: 'normal'
        },
        {
            name: 'Open Sans',
            weight: 'normal',
            style: 'italic'
        },
        {
            name: 'Open Sans',
            weight: 'bold',
            style: 'normal'
        },
        {
            name: 'Open Sans',
            weight: 'bold',
            style: 'italic'
        }
    ];

    var html = document.documentElement;

    html.classList.add('fonts-loading');

    if (sessionStorage.fontsLoaded) {
        html.classList.remove('fonts-loading');
        html.classList.add('fonts-loaded');
        return;
    }

    Promise
        .all(fonts.map(function(font) {
            return (new FontFaceObserver(font.name, {
                style: font.style,
                weight: font.weight
            })).load();
        }))
        .then(function () {
            html.classList.remove('fonts-loading');
            html.classList.add('fonts-loaded');
            sessionStorage.fontsLoaded = true;
        })
        .catch(function () {
            html.classList.remove('fonts-loading');
            html.classList.add('fonts-failed');
            sessionStorage.fontsLoaded = false;
        });

})(window, document);

статьи-о-загрузке-шрифтов.md

1. https://web-standards.ru/articles/web-font-loading-patterns/
2. http://css-live.ru/articles/minimalno-neobxodimye-shrifty.html
3. http://css-live.ru/articles/antipattern-dlya-veb-shriftov-data-uri.html
4. http://css-live.ru/articles/ischerpyvayushhee-rukovodstvo-po-strategiyam-zagruzki-veb-shriftov.html