HTTP request failed: null

This error is not an HTML validation issue. It is a network connectivity problem between your browser (or the W3C validator tool) and the server it is trying to reach.

The W3C Markup Validation Service at https://validator.w3.org needs to fetch your document before it can check it. When you see "HTTP request failed: null", the validator could not establish a connection to retrieve the page you submitted. This typically happens when:

• You submitted a URL pointing to localhost or a private IP address (e.g., 127.0.0.1, 192.168.x.x, 10.x.x.x). The W3C validator runs on a remote server and cannot access your local machine.
• The target server is behind a firewall, VPN, or requires authentication.
• The URL is unreachable because the server is down or the domain does not resolve.
• A temporary network issue prevented the validator from connecting.

How to fix it

If your site is only running locally, use the "Validate by Direct Input" option on the W3C validator. Paste your HTML source code directly into the text area at https://validator.w3.org/#validate_by_input instead of providing a URL.

If your site is publicly hosted, confirm that the URL is correct, the server is running, and no firewall rules block incoming requests from external servers. Try opening the URL in a browser to verify it loads. Then resubmit it to the validator.

You can also install the vnu-jar (Nu Html Checker) locally to validate pages on your own machine without needing a public URL:

java -jar vnu.jar http://localhost:3000/my-page.html

This runs the same validation engine the W3C uses, but directly on your network.

Learn more:

MDN Reference

Troubleshooting HTML validation

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W3C Reference

W3C Markup Validation Service

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