A browser can decide whether markup forms one properly nested XML document. It cannot infer the vocabulary, data types, required relationships, signature trust, or operational meaning expected by the system that receives it.
What well-formedness checks
XML 1.0 requires one document element, matching start and end names, proper nesting, quoted attribute values, unique attribute names, legal characters, and correct entity syntax. A fatal violation stops normal parsing. XMLClear exposes that browser parser boundary and the first line and column when the browser reports them. It does not guess a missing close tag or silently treat XML as forgiving HTML.
What validity adds
A DTD or schema can define which element names are allowed, their order and cardinality, attribute defaults, data types, and additional constraints. An application can impose still more rules: identifiers may need to exist elsewhere, totals may need to agree, a signature may need a trusted certificate, or a message may need a specific protocol version. A document can satisfy XML syntax and fail every one of those contracts. XMLClear therefore uses ‘well-formed’ and never labels the result schema-valid, trusted, secure, signed, or ready for production.
A document can parse and still fail a schema
For example, <count>many</count> is well-formed XML. It fails an XSD rule requiring count to contain an integer. In contrast, <count>3</total> fails parsing because the closing name differs. XMLClear checks syntax, not the hypothetical schema or the meaning of the value.