A fallback for stylesheets loaded with prefetch

stylesheet-prefetch-fallback.js

var DOMTokenListSupports = function (tokenList, token) {
    if (!tokenList || !tokenList.supports) {
        return;
    }
    try {
        return tokenList.supports(token);
    } catch (e) {
        if (e instanceof TypeError) {
            console.log("The DOMTokenList doesn't have a supported tokens list");
        } else {
            console.error("That shouldn't have happened");
        }
    }
};

// test if prefetch is supported
const linkSupportsPreFetch = DOMTokenListSupports(document.createElement("link").relList, "prefetch");
if (!linkSupportsPreFetch) {
    const prefetchLinks = document.querySelectorAll('link[rel=prefetch]');

    for (i = 0; i < prefetchLinks.length; i++) {
        let prefetchLink = prefetchLinks[i];

        let linkType = prefetchLink.getAttribute('as');
        let linkOnload = prefetchLink.getAttribute('onload');

        if ('style' === linkType) {
            prefetchLink.setAttribute('rel', 'stylesheet');
            prefetchLink.removeAttribute('as');
        }
        if ('' !== linkOnload) {
            prefetchLink.removeAttribute('onload');
        }
    }
}