Practical guidance
Preserve source encoding and declared namespaces. Ampersands in text generally need escaping. Do not enable external entity loading for untrusted documents because this creates a separate server-side XML parsing risk.
Before using this tool
Define what you need from XML validator and prepare the relevant input. When diagnosing a live service, record the test time and connection conditions so you can compare results after configuration changes. Check findings against the documentation for your software.
Tool availability
This is currently an information page. No working form, calculations or submission to diagnostic services is available yet. The interface and supported input formats will be documented when functionality is released.