What it checks
The input must be well-formed: one root element, every tag closed in the right order, quoted attributes, declared namespace prefixes and valid entities. The first problem is reported with its line and column. It does not validate against an XSD schema.
For safety the parser never loads DTDs or external entities (the classic XXE and "billion laughs" attacks). Comments and processing instructions are dropped from the formatted output; CDATA content is kept as text.
Is my data uploaded?
No. Formatting runs in your browser, so nothing you add is sent to gratistools.be. See the privacy page for details.