Document syntax
Document type declaration
The <!DOCTYPE root …> in the prolog that names the document's root element and points at, or contains, its markup declarations.
A document type declaration does two separable jobs, and conflating them causes most of the confusion around it. It can point at an external DTD by SYSTEM or PUBLIC identifier, and it can carry an internal subset in square brackets. The internal subset is where entity declarations live, which is why a DOCTYPE turns up in documents that have nothing to do with validation.
Nothing about a DOCTYPE makes a parser validate. Validation against a DTD is a separate mode that has to be asked for, and most parsers — ours included — do not run it. A document can declare a DTD it flagrantly violates and still be read without complaint.
- Defined in
- XML 1.0 §2.8
- Category
- Document syntax
- Also written
- DOCTYPE, <!DOCTYPE>
Shown, not asserted
All 2 claims are executed on every test run, against the same engines the tools use. If the behaviour changes, the test fails rather than this page quietly becoming wrong.
A document that violates its own DTD is still well-formed: the element model is not checked.
document.xml <!DOCTYPE note [<!ELEMENT note (to)><!ELEMENT to (#PCDATA)>]> <note><from>who</from></note>What the engine returns
Well-formed.The internal subset is read for entity declarations, and those are expanded.
document.xml <!DOCTYPE x [<!ENTITY who "Ada">]> <x>Hello &who;</x>expression string(/x)What the engine returns
Hello Ada
What catches people out
- A DOCTYPE is not a schema reference. xsi:schemaLocation is the XSD equivalent, and it is a hint rather than an instruction too.
- The root element named in the DOCTYPE must match the document's actual root, and that much even a non-validating parser will check.
- Many hardened parsers reject DOCTYPE outright, because it is the doorway to external entity and entity expansion attacks. Emitting one can make your document unreadable to a careful consumer.
Where it bites
A definition on its own is trivia. These are the pages where this term decides whether a document works.
- Parse errorsEntity is not definedWhat referencing an entity without declaring it in the internal subset reports.
- ToolsXML validatorReports well-formedness — which is precisely what a DOCTYPE alone does not buy you.
Related terms
- Internal subsetThe markup declarations written inline between square brackets in a DOCTYPE, rather than fetched from an external DTD file.
- General entityAn entity declared with <!ENTITY name "text"> whose replacement text is substituted wherever &name; appears in content.
- External entityAn entity whose replacement text lives outside the document, named by a SYSTEM or PUBLIC identifier and fetched when the entity is referenced.
- DTDThe original XML schema language: element and attribute declarations written in their own non-XML syntax, plus the entity declarations nothing else provides.
- ValidA well-formed document that also conforms to a declared grammar — a DTD, an XSD schema or a RELAX NG grammar.
- XXE (XML external entity attack)An attack in which a submitted document declares an external entity so that a trusting parser reads a local file or makes a request on the attacker's behalf.
Get started
Bring order to the XML your team can't afford to ignore.
Create a free account and get a private workspace to search, validate, diff, and monitor your XML feeds, sitemaps, schemas, and vendor integrations.