Sitecore XHTML Validator failures and how to troubleshoot them

Many Sitecore implementations rely on the out-of-the-box XHTML validator, available at: /sitecore/system/Settings/Validation Rules/Item Rules/Item/Full Page XHtml.

However, this validator can fail in scenarios where the rendered page is fully valid, returning a 500 error without providing useful diagnostic information. Recent testing revealed that the issue lies not with the content itself, but with the validator method, which fails silently and lacks proper error reporting.

How the XHTML Validator Works

1. Constructs a friendly URL for the current item, including additional parameters for language, device, and related contexts.
2. Initiates a web request to retrieve the rendered page using the constructed URL. This is typically where failures occur.
3. Processes the response and submits it to the W3C Validator. While this part contains additional logic, failures at this stage are generally handled more gracefully.
4. Returns validation results back to the user.

Why Does It Break?

If the web request fails for any reason, the validator returns an error without meaningful details. The exception message is exposed, but the more relevant inner exception is not. Additionally, nothing is logged to the Sitecore logs, making diagnosis more difficult.

Troubleshooting the XHTML Validator

Failures at this stage can be caused by several factors. In the case examined here, the root cause was a missing or incorrect proxy configuration.

Consider the following checks to diagnose and resolve similar issues:

• Confirm that all required SSL certificates are valid and trusted. Untrusted or expired certificates will cause failures.
• Verify that server proxy settings are correctly configured to allow outbound web requests.
• Ensure that URL rewriting rules are not interfering with the validator’s requests. Unexpected rewrites can break the validation process.
• If the issue persists, consider creating a custom implementation of the Sitecore.Data.Validators.ItemValidators.FullPageXHtmlValidator. Update the validator item in Sitecore to use this new implementation and debug it directly.
• If XHTML validation is not a critical requirement for your solution, disabling the validator may be a viable option.

Proper logging and error handling in this area of Sitecore remain limited. Customizing the validator can provide better insight and control over the validation process, particularly in complex deployment environments.