XML files are validated against their DTD or XSD schema. If no schema can be loaded, only minimal error checking will be performed. No validation of any kind is performed for HTML files, but XHTML files can be validated using their regular schema.

Any errors found while parsing XML are handled by the ErrorList plugin; in other words, they are highlighted in the text area, and shown in the Plugins>Error List>Error List window. See the documentation for the ErrorList plugin for details.

DTDs (document type definitions) and XSDs (w3c XML Schemas) are two different standards for defining XML Schemas, to describe which elements and entities are allowed within a specific context. In order to implement completion or validation, the XML plugin needs to be able to load a schema. A few file types for which no schema is available are supported using built-in completion information included with the plugin.