HTTP request failed: java.net.SocketTimeoutException: Connect Timeout

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 webpage over HTTP to check it. A SocketTimeoutException: Connect Timeout means the validator's server could not establish a connection to your URL within its time limit. Common causes include:

• The target server is down or unreachable from the validator's network.
• A firewall or security group blocks incoming requests from external servers.
• The server is behind a VPN, on localhost, or on a private network that the validator cannot access.
• DNS resolution fails for the provided URL.
• The server takes too long to respond, exceeding the validator's timeout threshold.

How to work around it

If the validator cannot reach your site, validate the HTML directly instead of providing a URL. Copy your HTML source and paste it into the "Validate by Direct Input" tab on https://validator.w3.org/#validate_by_input. You can also use the "Validate by File Upload" option to upload an .html file from your machine.

For local development, the Nu Html Checker can run locally as a standalone Java application or Docker container, which removes the need for any external network access:

java -cp vnu.jar nu.validator.servlet.Main 8888

This starts a local instance of the validator on port 8888, and you can validate pages hosted on localhost without timeout issues.

Learn more:

MDN Reference

Troubleshooting network issues - MDN Web Docs

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