Try each link on the page, and if it's dead, made it red

redlinks.js

// https://gist.github.com/jkoop/577a96fa421a8dfec22a48334a7493a0

function scanForRedLink(link, url) {
    if (link.classList.contains('no-redlink')) { // for stuff like logout links
        return;
    }

    if (url === null || url.startsWith('javascript:') || url.startsWith('#') || url.length < 1) { // for stuff like javascript and elements
        return;
    }

    fetch(url, {
        method: 'HEAD',
        mode: 'no-cors',
        redirect: 'follow',
    }).then(function (response) {
        console.log(url, response.status);

        // if the response is 401 or 403, we don't know if the page exists or not
        if (response.status < 200 || response.status >= 404) {
            link.style.color = 'red';
        }
    });
}

function scanForRedLinks() {
    var links = document.getElementsByTagName('a');

    for (var i = 0; i < links.length; i++) {
        var link = links[i];
        var url = link.getAttribute('href');

        scanForRedLink(link, url);
    }
}

document.addEventListener('DOMContentLoaded', scanForRedLinks);