font-defer.js

(function() {
    'use strict';

    var files = ['assets/site/css/non-critical.css', 'assets/site/img/icons.svg'];

    function getExtension(src) {
        return src.split('.').pop();
    }

    function onEvent(el, event, callback) {
        if (el.addEventListener) {
            el.addEventListener(event, callback);
        } else if (el.attachEvent) {
            el.attachEvent('on' + event, callback);
        }
    }

    function injectContent(content, type) {
        if (type === 'css') {
            var style = document.createElement('style');
            document.getElementsByTagName('head')[0].appendChild(style);
            if (style.styleSheet) {
                style.styleSheet.cssText = content;
            } else {
                style.innerHTML = content;
            }
        } else if (type === 'svg') {
            document.getElementsByTagName('head')[0].insertAdjacentHTML('beforeend', content);
        }
    }

    function loadFile(file) {
        var content = localStorage[file];
        if (content) {
            injectContent(content, getExtension(file));
        } else {
            onEvent(window, 'load', function() {
                var xhr = new XMLHttpRequest();
                xhr.open('GET', file, true);
                onEvent(xhr, 'load', function() {
                    if (xhr.readyState === 4) {
                        injectContent(xhr.responseText, getExtension(file));
                        localStorage[file] = xhr.responseText;
                    }
                });
                xhr.send();
            });
        }
    }

    if (window.localStorage) {
        for (var i = 0, count = files.length; i < count; i++) {
            if (getExtension(files[i]) === 'svg' && !document.implementation.hasFeature('http://www.w3.org/TR/SVG11/feature#Image', '1.1')) {
                return;
        }
        loadFile(files[i]);
       }
    }

})();