4. XML universality: namespaces and schemas

4.1 A valid document?

For a document to be declared "valid", i.e. free of errors and therefore of maximum compatibility, it must of course validate the formalism prerequisite: the document must therefore be "well-formed", along the lines of what was discussed earlier. There are several methods for validating syntax and tag nesting issues. A simple and official tool is the W3C online validator ( https://www.w3schools.com/xml/xml_validator.asp ), which will check these aspects point by point. For users of Linux-based operating systems (including Mac OS), a good command-line verification tool is xmllint. Source code editors are not equal when it comes to error detection, but...