HTML Guides for page not found

This error is not about your HTML syntax being invalid — it’s about a broken reference. The W3C Validator follows URLs it encounters in your markup (or a URL you submit directly for validation) and checks whether the server can actually deliver the resource. When the remote server returns an HTTP 404 (Not Found) response, the validator flags the issue because the referenced resource is missing or unreachable.

There are several common causes for this error:

• Typos in the URL — A misspelled filename, path, or domain name.
• Moved or deleted resources — The file existed at one point but has since been removed or relocated.
• Case sensitivity — Many web servers treat Image.png and image.png as different files. A mismatch in letter casing can produce a 404.
• Incorrect relative paths — A relative URL that resolves differently than expected based on the document’s location.
• External resources no longer available — Third-party CDNs or hosted files that have been taken down.

This matters because broken references degrade the user experience. Missing stylesheets can leave a page unstyled, missing scripts can break functionality, and missing images display broken image icons. Search engines also penalize pages with excessive broken links, and screen readers may announce confusing or unhelpful content when resources fail to load.

How to Fix It

1. Check the URL carefully. Copy the full URL from your HTML, paste it into a browser, and see if it loads. If it returns a 404 page, the URL is wrong.
2. Verify the file exists on the server. If you control the server, confirm the file is in the expected directory with the exact filename and extension.
3. Fix case sensitivity issues. Ensure the capitalization in your URL matches the actual filename on the server.
4. Update moved resources. If a file was relocated, update the href or src attribute to point to the new location.
5. Replace unavailable external resources. If a third-party resource is no longer available, find an alternative source, host a copy yourself, or remove the reference.

Examples

Broken image reference (triggers the error)

<img src="https://example.com/images/photo.jpeg" alt="A scenic landscape">

If photo.jpeg doesn’t exist at that path (perhaps the actual file is named photo.jpg), the validator will report a 404 error.

Fixed image reference

<img src="https://example.com/images/photo.jpg" alt="A scenic landscape">

Broken stylesheet reference (triggers the error)

<link rel="stylesheet" href="/css/Styles.css">

If the file on the server is actually named styles.css (lowercase), a case-sensitive server will return a 404.

Fixed stylesheet reference

<link rel="stylesheet" href="/css/styles.css">

Broken script reference with incorrect path (triggers the error)

<script src="/assets/js/old-directory/app.js"></script>

If the script was moved to a different directory, this path no longer resolves.

Fixed script reference

<script src="/assets/js/app.js"></script>

Using a relative path incorrectly (triggers the error)

If your HTML file is at /pages/about.html and you reference an image like this:

<img src="images/logo.png" alt="Company logo">

The browser will look for /pages/images/logo.png. If the image actually lives at /images/logo.png, this will fail.

Fixed with a root-relative path

<img src="/images/logo.png" alt="Company logo">

The leading / ensures the path is resolved from the root of the site, regardless of where the HTML document is located.