ImgMultiSrcLoader.js

// ImgMultiSrcLoader.js

// Usage:
//      <img src="1.png" data-multi-src="1.png 2.png https://example.com/3.png">
window.addEventListener('load', function () {
    document.querySelectorAll('img[data-multi-src]').forEach(function (img) {
        const srcArr = img.getAttribute('data-multi-src').split(' ');
        srcArr.forEach(function (src) {
            const xhr = new XMLHttpRequest();
            xhr.addEventListener('load', function (e) {
                if ((e.target.status === 200 || e.target.status === 304) && img.getAttribute('data-multi-src-loaded') !== 'true') {
                    img.setAttribute('src', src);
                    img.setAttribute('data-multi-src-loaded', 'true')
                };
            });
            xhr.open('GET', src);
            xhr.send();
        });
    });
});