Broken links harm SEO in three connected ways. First, crawl budget: search engines allocate a limited number of crawls per site, and every broken link sends the bot to a dead end instead of a real page — if 5% of your internal links are broken, roughly 5% of your crawl budget is wasted. Second, link equity: the ranking authority (“link juice”) a link would pass is lost when its destination is dead, so a high-authority page with two broken outbound links out of ten leaks about 20% of the equity it could distribute. Third, user experience: a visitor who clicks through and lands on a 404 usually hits the back button, driving up bounce rate, which can signal to search engines that the page isn’t satisfying intent. Google says isolated 404s are normal and not a direct penalty, but the indirect fallout — wasted crawl budget, leaked equity and poor UX — is what erodes rankings over time.

A browser genuinely cannot reliably check whether an external link is broken, and this tool is honest about that. Cross-origin requests are blocked by the browser&rsquo;s Same-Origin Policy / CORS, so JavaScript can&rsquo;t read the HTTP status of another site&rsquo;s URL; a <code>no-cors</code> fetch returns an OPAQUE response with status <code>0</code> and no readable headers, so a failed request cannot be told apart from a real 404. We also use no server proxy, which keeps your URLs private. What the tool does reliably instead is EXTRACT your links, VALIDATE their URL format (catching the malformed, mis-typed and placeholder URLs that are broken on their face), flag the suspicious ones, and give you an &ldquo;Open ↗&rdquo; link to verify each one manually. The optional reachability probe attempts a real fetch but reports cross-origin failures as &ldquo;blocked (CORS)&rdquo; — inconclusive, not proof of a 404.

Dedicated link checkers such as Screaming Frog SEO Spider, paid SEO suites Site Audit, Xenu&rsquo;s Link Sleuth and the W3C Link Checker run server-side (or as a desktop app), outside the browser&rsquo;s cross-origin sandbox, so they are not bound by CORS. They crawl your pages, extract every link, then send an HTTP request — usually a HEAD (or a GET when HEAD is rejected) — to each target URL and read the actual response status code, classifying anything in the 4xx range (like 404 Not Found) or 5xx range (server errors) as broken. They can also report the source page (&ldquo;inlinks&rdquo;) so you know exactly where to fix each dead link. Because they read the true status, they can confirm reachability in a way a browser tab cannot — which is why, for a true site-wide crawl, this tool points you to them.

The format validation flags the classic signs of a link that will break or is broken on its face: a MALFORMED URL the browser can&rsquo;t parse; a MISSING PROTOCOL (a bare &ldquo;example.com/path&rdquo; that may be treated as a relative path); SPACES or other whitespace inside the URL (browsers may not follow it); an <code>http://</code> link on an <code>https://</code> site (mixed content, often blocked); <code>javascript:</code> pseudo-URLs, empty hrefs and bare <code>#</code> placeholder links that go nowhere; <code>localhost</code> or private-IP links that work only on your machine and are dead in production; un-filled placeholders or template leftovers (<code>undefined</code>, <code>null</code>, <code>{{product_url}}</code>, <code>example.com</code>); overly-long URLs; and DUPLICATE targets. Each issue is explained in plain language and rolled into an overall valid / suspicious / invalid rating so you can fix the obvious problems before running a full crawl.